EARLY MAMMALIAN DEVELOPMENT. 



99 



Between the mesodermal somites and the cleft portion of the mesoderm a 

 small group of cells represents the intermediate cell mass or nephrotome. 



The formation of the neural plate and tube from the axial band of ecto- 

 derm is quite similar to its development in the lower forms. Subsequently 

 the disk is bent or rolled into the typical cylindrical vertebrate body, a pro- 

 cess described in the chapter on "External Form" (p. 109 et seq.). 



The Germ Layers in Man. 



There are no observations on the development of the human ovum prior 

 to the appearance of all three germ layers. Consequently nothing is known 



tro. 



cyt. P.e. 



tro. n.z. 



tro. 1 tro. 1 



FIG. 73. Section of a human ovum of about 14 days, embedded in the uterine mucosa. 



Bryce and Teacher. 



Cap., Capillary; cyt., cellular layer (cyto-trophoderm) ; ep., uterine epithelium; gl., uterine 

 gland; n.z., necrotic zone of decidua (uterine mucosa); P.e., point of entrance of the 

 ovum; tro., syncytial layer (plasmodi-trophoderm) ; tro. 1 , masses of vacuolating syncytium 

 invading capillaries. The cavity of the vesicle is filled with mesoderm in which are em- 

 bedded the amniotic cavity (the larger) and the yolk cavity. 



concerning fertilization, cleavage, the first differentiation of cells, the forma- 

 tion of the embryonic disk, or the mode of origin of the germ layers. In the 

 youngest human embryo that has been recorded, the one described by Bryce 

 and Teacher in 1908, all three germ layers are already present. The age of 

 this embryo was reckoned to be about 14 days. 



In certain respects the Bryce-Teacher embryo (Fig. 73) bears fundamental 

 resemblances to corresponding stages of lower mammals, especially the lower 

 primates; in other respects there are differences which are not irreconcilable, 



