74 THE EARLY DEVELOPMENT OF MAMMALS. 



flattened from side to side, and therefore acquires a characteristic oval configura- 

 tion as seen in cross-section (Fig. 28). We can now recognize in the cross-sections 

 four regions : first, the two thick sides; second, in the median dorsal line the thin 

 portion which we call the deck-plate, and in the median ventral line the thin por- 

 tion which we call the -floor -plate. Later on, each lateral portion becomes sub- 

 divided into two longitudinal bands, known as the zones of His, and distin- 

 guished from one another as the dorsal and ventral zones. After this stage there 

 are six longitudinal zones in the embryonic cord. These are, first, the deck- 

 plate; second and third, the dorsal zones of His; fourth and fifth, the ventral 

 zones of His ; and sixth, the floor-plate. These six zones also appear in the region 

 of the brain, where, however, they undergo characteristic modifications. The 

 zones of His dominate the entire morphology of the central nervous system. 



The Early History of the Mesoderm. 



Concerning the precise origin and early development of the mesoderm au- 

 thorities are by no means agreed, and in the interpretations offered there has 

 been more of hypothesis than of observation. The most accurate observations 

 have so far been made on the elasmobranchs, lizards and chick. In these forms 

 the entoderm (or segmenting yolk) in the neighborhood of the primitive streak 

 produces cells which take their place so as to form a layer next to the entoderm. 

 This layer gradually becomes more and more distinct until it can be definitely 

 recognized as a separate layer, the mesoderm. It is probable that a similar 

 process goes on in amphibia and in mammals, so that it is safe to say that the 

 mesoderm probably arises by this process, which we call delamination, in all 

 vertebrates. In its first stage the mesoderm has no distinct boundary against 

 the underlying entoderm. It is thickest in the neighborhood of the primitive 

 streak and thins out from that in all directions. It very early comprises two 

 easily recognizable classes of cells. One of these forms a more or less distinct 

 layer next to the yolk, and so distributes itself as to form a network of cavi- 

 ties of which these cells become the boundaries, thus developing the first blood- 

 vessels. The cells which form them constitute the angiohlast. A portion of the 

 angioblastic cells comes to lie in the cavities of these primitive blood-vessels and 

 is transformed into the. first red blood-corpuscles of the embryo. The second 

 class of cells constitutes the mesoderm proper, and forms a more continuous 

 sheet of undifferentiated, somewhat closely compacted cells, extending out from 

 the primitive streak and lying between the angioblast and the ectoderm. 



The Expansion of the Mesoderm. — After the mesoderm is once formed as a 

 distinct layer, it seems to have no longer any connection with the entoderm or 

 ectoderm, except in the axial line. Its further expansion is due to the prolifera- 



