EMBRYOLOGY 25 



at the margin. A small space between the blastoderm and the 

 yolk represents the segmentation cavity. Additional marginal cells 

 continne to form in such a manner that they eventually surround 

 the yolk, but never take part in the formation of the embryo itself. 

 Gastrulation is accomplished by an ingrowth of cells along the 

 ])osterior margin of the blastoderm. (Fig. 4.) Following endoderm 

 formation, a ])rimitive streak appears on the blastoderm as a linear 

 thickening in which a shallow depression, the primitive groove, 

 appears. This groove is believed to be homologous with the blasto- 

 pore of Amphioxus. Mesoderm arises from lateral proliferations of 

 cells of the primitive streak which form plates that grow laterally 

 between the ectoderm and endoderm on either side of the mid-line. 

 The plates split distally, thus forming the somatopleure and the 

 splanchopleure with ectoderm and endoderm, respectively. The 

 coelome is the space between somatic and splanchnic mesoderm. 



Mammalia.— Although the eggs of mammals possess a minimvun 

 of yolk, the development of the embryo does not follow the simple 

 plan outlined for the egg of Amphioxus. Development of the 

 mammal is complex and possibly modified because of evolution 

 from an ancestral condition in which yolk was abundant in the egg, 

 a condition still present in Monotremes. Yolk has ])ractically dis- 

 appeared from most mammalian eggs and the zygote developing 

 within the parental body secures nutritional supplies from maternal 

 tissues. Such modifications change the developmental processes 

 by which the embryo becomes established. 



Cleavage is complete and the early stages superficially resemble 

 those of Amphioxus. Later clea^'age results in the formation of an 

 inner cell mass surrounded by a single layer of cells, called the 

 trophectoderm, which is in contact with the wall of the uterus of 

 the mother. 



The trophectoderm grows more rapidly than the inner cell mass 

 which is left attached only at one point on the trophectoderm wall. 

 The cavity between the two regions is filled with fluid and is homolo- 

 gous with the segmentation cavity of lower forms. This stage is 

 recognized as a specialized blastula and is called a bJastocj/st. The 

 embryo jjroper develops from the inner cell mass which is homolo- 

 gous with the disc-like blastoderm of the reptiles and birds. Cells 

 derived from the lower surface of the inner cell mass form endoderm 

 and the remainder of the mass becomes ectoderm. 



Eventually a primitive streak appears on the blastocyst and is 

 morphologically similar to that in Sauropsida. A primitive groove 

 also appears which is homologous with the blastopore of lower 



