It was assumed that organizer cells existed. They have been shown on chicken embryos:

https://www.the-scientist.com/?articles.view/articleNo/54640/title/Animals--Embryonic-O...

"The organizer, a group of cells in the embryo that directs the developmental fates and morphogenesis of other embryonic cells, has been identified in human tissue for the first time, according to a study published today (May 23) in Nature. The discovery demonstrates that the organizer is evolutionarily conserved from amphibians to humans.

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"Rockefeller University embryologist Ali Brivanlou and colleagues report that when they grafted human stem cells that they’d treated with Wnt and Activin, two signaling proteins previously shown to be involved in organizer gene expression in other animals, into chick embryos, the grafted cells set off the developmental progress of the cells around them. The experiment establishes for the first time that the organizer exists in humans and that Wnt and Activin work in concert to make it possible for cells to direct embryonic development.

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"In this latest experiment, when they grafted human embryonic stem cells that they treated with Wnt and Activin into chick embryos, the stem cells caused the cells around them to begin forming a second neural axis as a line of cells running along one side of the embryo. This effectively demonstrated that Wnt and Activin signaling trigger some of the cells in early human embryos to become the organizer.

“'The beautiful thing about this is that [Brivanlou] . . . has really used cutting-edge approaches to demonstrate that this idea of the organizer, and specifically the genetic and molecular mechanisms that have been described across animal systems, can be employed in the human system,” says Daniel Kessler, a developmental biologist at the University of Pennsylvania who was not involved in the study. “This is as close as we’ll get to a definitive demonstration that these principles and mechanisms apply in the human embryo.”

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"But the discovery also has deeper meaning to him. “Human beings have always been interested in their own origins,” he says. “The amazing ability that we now have to look at our earliest moments of development and genesis is something that, to me, is as attractive—if not more—as looking at the pictures from the Hubble telescope.'”

Comment: For me the process of embryology science shows how complex it is to make an individual from a single egg. Only a desinger could create this process.

In rat embryology the thalamus guides nerve pathways to the body surface from the forming sensory cortex . It is assumed the same occurs in humans:

https://www.sciencedaily.com/releases/2019/05/190503100758.htm

"All the surface of the human body is represented in the cerebral cortex in a transversal band localized at the external part of the cerebral hemispheres: the somatosensory cortex. Each body region occupies in that band a distinct extension depending on its use and sensitivity. For instance, the hands and the lips, on which humans rely most, occupy the largest area. Thus, the 3D representation of that map forms the known sensory homunculus.

"Similar to a cartographic map, each represented region of the body in the somatosensory cortex is connected to its corresponding body surface thanks to the neuronal pathways that keep a strict topographical relationship along the nervous system. In this pathway, the thalamus, a deep structure of the brain that lies beneath the cortices, plays a key role by relaying the peripheral information to the cortex without losing the point-to-point correspondence. Using this extraordinary precision, we can discriminate which point of our body is receiving an external stimulus and have a well-defined map of the periphery. Such accurate topography constitutes the basis of the sense of touch and is essential for survival of the species.

"How are the neurons of the somatosensory cortex organized during development to perform these functions? Neurons of this brain region, as in the rest of the cerebral cortex, are assembled into columns that are placed next to each other like building bricks. It remains unknown, however, how these columnar structures become functional correspondents of the distant regions of the periphery.

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" Far from being a mere relay station, the thalamus guides the formation of the functional cortical columns and the concomitant somatotopic map in the still immature cortex, before external sensory experience is an effective source of information. In particular, the thalamus does so by generating and transmitting patterns of spontaneous activity (called waves) to the developing cortex. The discovery was made in rodents, in a particular and extensive region of their somatosensory cortex: the barrel cortex. This area contains the representation of the whiskers of the snout that are, for rodents, sensorially equivalent to our hands.

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"'It is very probable that this mechanism involved in the formation of the sensory maps that we have discovered in rodents can be extrapolated to humans, because the organization of the cortex is evolutionarily conserved between species," explains López-Bendito.

"'The spontaneous activity of the thalamus is not something circumstantial, but contains important information for the construction of the brain during embryonic development. It was previously thought that neuronal circuits were built on a genetic imprint and that the postnatal sensory experience ends up defining the maps. This work questions this vision because it demonstrates the existence of these maps before birth,"; says Lopez-Bendito. "Our results indicate that the spontaneous thalamic activity during the embryonic phase is essential for the normal development of the brain, defining what in neurobiology is called a critical period, i.e. a period of time in which plastic changes are possible but after which alterations would be irreparable"; she adds." (my bold)

Comment: Arranging for maps of the body surface in the brain's cortex through information in the genome requires coordinated planning and requires complex design to develop. The thought that cellular intelligence is capable of doing this is unreasonable. Cellular intelligence can reasonably make minor adaptations within species, no more.

Certain proteins from the mother control patterns of development in teh egg:

https://phys.org/news/2020-07-complex-developmental-patterns-surprisingly-simple.html

"Proper embryonic development of the fruit fly Drosophila melanogaster is governed by patterns of protein activity bequeathed to the fertilized egg by its mother. While the embryo is still a single cell, the maternal cells surrounding it deposit certain proteins inside it at specific locations. This establishes protein gradients that direct the development of embryonic features along its anterior-posterior and ventral-dorsal axes. Later, the embryo receives another round of maternal information, called terminal patterning, that guides the development of its head and tail.

"Terminal patterning is driven by a protein called Torso that is made by the mother and deposited throughout the embryo. Torso is stimulated by binding to other proteins that are also produced by the mother, but present only at the embryo's anterior and posterior ends. Torso stimulation kicks off several signaling cascades, including one called the Ras/ERK pathway, whose activation directs the expression of genes crucial to embryonic development. Mothers lacking Torso or the proteins to which it binds are sterile because their embryos fail to develop head and tail.

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"Much of terminal patterning is driven by two genes whose expression is controlled by the Ras/Erk pathway in response to Torso signaling. In normal embryos, the expression of these two genes occurs in different cells at different times. In light-stimulated OptoSOS embryos lacking Torso signaling, however, expression of the two genes overlapped more in both space and time, suggesting their precise expression patterns are not required for development.

"The authors next investigated whether different aspects of the developmental program are triggered at the same or different stimulus thresholds. To do this, they monitored embryonic development after varying the intensity and duration of light stimulus in OptoSOS embryos lacking Torso signaling.

"'We found that the terminal pattern appears to work as a series of switches, where successively longer light pulses trigger a predictable sequence of body parts being 'rescued' one by one," explains Toettcher.

"Together, these data suggest that for terminal signaling, what appears to be a very complex developmental program is actually under the control of a relatively simple system that depends on different thresholds of Ras/Erk signaling."

Comment: Bit by bit we are learning how embryological systems work, by design.