EARLY MAMMALIAN DEVELOPMENT. 



95 



a and b) and in the white rat (Fig. 63) point to the conclusion that the meso- 

 derm originates from the ectoderm. 



In the primates thus far studied the mesoderm develops before the 

 primitive streak appears in the embryonic disk. In Tarsius spectrum the 

 .middle germ layer appears between trophoderm and entoderm while the disk 

 is still composed only of the two primary layers (Fig. 67). In Semnopithecus 

 nasicus a similar condition obtains (Fig. 68). In the human ovum de- 

 scribed by Bryce and Teacher the mesoderm is an extensive layer when the 



FIG. 67. Median longitudinal section through the embryonic disk and yolk sac of the lemur, 

 Tarsius spectrum. Hubrecht, from Quain's Anatomy, emb.ect., Embryonic ectoderm; 

 mes, mesoderm; pp, entoderm; y.s., yolk sac. 



germ disk is still rudimentary (Fig. 73). The origin of the mesoderm in 

 these cases is problematical. In the lemur Hubrecht maintains that it is 

 derived from the ectoderm. In Semnopithecus Selenka considers it to be of 

 entodermal origin. The mesoderm of the embryonic disk itself appears 

 somewhat later in the same manner as in the lower mammals, that is, from 

 the thickened ectoderm along the primitive streak (Fig. 69). 

 J Following the development of the primitive streak in the dog's disk the 

 disk becomes oval, and anterior to the streak an opaque band appears which 

 is comparable to the primitive axis of the bird's blastoderm (Fig. 70, a\ 



