266 Basic Structure of Vertebrates 



Fig. 221. Transverse section of the embryonic shield of a rabbit at the stage 

 represented in Fig. 220. The section is taken at the position indicated by the line 

 S-S in Fig. 220. (EC) Ectoderm; (EN) endoderm; (MES) mesoderm; (PG) primi- 

 tive groove of primitive streak. ( X 175.) (After Assheton. Courtesy, Neal and 

 Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



of the absence of a large yolk-mass in the mammal. These facts point 

 to the conclusion that the developmental behavior of the reptilian em- 

 bryo had become so strongly established in the protoplasm of ancestral 

 reptiles and primitive mammals that it persisted even though the re- 

 duction of yolk had removed the immediate necessity for many of its 

 peculiarities — e.g., the yolk is reduced to a minimum, but a yolk-sac- 

 still persists. The many millions of years of primitive mammalian and 

 reptilian lineage constituted a barrier quite impassable by any possible 

 tendency for reversion to the indefinitely more remote developmental 

 methods of primitive Amphioxus-like chordates. 



Yet some primitive features persist. There can be little doubt that 

 the blastoporal growth-zone or germ-ring of Amphioxus and amphib- 

 ians and the primitive streak of reptiles are genetically continuous. If, 

 as seems very likely, the axial thickening of the mammalian embryonic 

 shield corresponds to the reptilian primitive streak — with even a mam- 

 malian relic of a blastopore — then the manner of origin of the meso- 

 derm is essentially similar from primitive chordate to mammal. 



Unquestionably the yolk content of the chordate egg is much more 

 readily subject to evolutionary change than is the developmental 

 mechanism of the germinal protoplasm. That mechanism can be 

 changed, but there is a high degree of inertia about it. The embryo — 

 i.e., the living protoplasmic thing exclusive of inert yolk — is highly 

 conservative. It tends to follow the old methods and it changes only 

 as it must. It adheres to old processes which, under new circumstances, 

 are indirect and unnecessarily complicated. 



ORGANOGENESIS 



The earlier period of development is concerned with laying out the 

 building materials, the embryonic or "germ" layers. In the later and 

 longer period, these layers are shaped into organs. The formation of the 

 central nervous organs and the notochord may begin, however, before 

 the mesoderm is fully established. Amphioxus. partly because it is so 

 small and partly because it is in so many respects primitive, affords 



