distinguishing the two layers of the lateral plates of the 

 mesoderm, which are now named somatic and splanchnic 

 mesoderm. (97) 



Baer also related the separation of the vertebrate 

 central nervous system rudiment into layers to primary 

 differentiation. Baer remarked that the skin and nervous 

 layers originate from the upper surface of the embryo, so his 

 point of view concerning the source of the central nervous 

 system remains unclear. 



The second type of differentiation, histological dif- 

 ferentiation, already occurs inside the layers, as a result 

 of which the skeleton, muscles, nervous system, and blood 

 form. 



The third type of differentiation, the morphological, 

 produces the external shape of the embryo. After that 

 occurs the embryonic layers are transformed into tubes; the 

 separated parts of these tubes acquire different configura- 

 tions and serve as a source for the organs. Thus, the neural 

 tube subdivides into the spinal and cephalic membranes and 

 the organs of sense. The mucous tube gives rise to the 

 mouth cavity, ^ the esophagus, stomach, intestine, respiratory 

 organs, liver, allantoides, and so on. Morphological 

 differentiation takes place, according to Baer, due to the 

 irregular growth of individual parts of the nervous, mucous 

 and other tubes. By irregular growth, Baer could explain 

 the apparent demarcation of one section of the tube from 

 another; for example, the divisions of the brain from each 

 other, the stomach from the intestine, and also the localized 

 protrusions giving rise to the sensory organs, respiratory 

 organs, liver, and allantoides. 



The three forms of differentiation described represent, 

 according to Baer, a source of heterogeneity of the organism. 

 Early in development, heterogeneity of the organs and their 

 constituent histological elements is less marked. "Observa- 

 tions," Baer said, "show more than any description can that 



5. It was not known to Baer that the mucous membrane of 

 the mouth cavity originates from the ectoderm. 



344 



