DECIDUAL MEMBRANES 



367 



present the diagram Fig. 368, A, followed by the diagrams B and C which 

 include many features actually observed. 



In Fig. 368, A, a mass of cells (ect.} represents the ectoderm which will 

 later cover the body and line the inner membrane or amnion. This ecto- 

 derm probably arises in connection with the layer (tr.) which covers the 

 entire vesicle and becomes the epithelium of the outer membrane or 

 chorion. The layer in question has been named the trophoblast (or 

 trophoderm) . 



ect. 



cho. 







PIG. 368. DIAGRAMS ILLUSTRATING THE EARLY DEVELOPMENT OF THE HUMAN EMBRYO (A is HYPO- 

 THETICAL). 



al., Allantois; am. c., amniotic cavity; cho., chorion; coe., coelom; ect., ectoderm; m, mesoderm; tr. 

 trophoderm (trophoblast); z, entodermal cyst; y. s., yolk-sac. 



The term trophoblast (i.e., nutritive layer) was introduced by Hubrecht to corre- 

 spond with the terms epiblast, mesoblast and hypoblast, which he used for the other 

 germ layers. Since these are now generally called ectoderm, mesoderm arid entoderm, 

 the outer layer should be trophoderm, and the substitution of this name is therefore 

 recommended. Trophoderm has, however, been used by Minot for the proliferating 

 part of Hubrecht's trophoblast. It may be noted that a similar difficulty is encoun- 

 tered in His's angioblast which, as a germ layer, should be angioderm. Schafer 

 applies angioblast logically to the individual cells which become the endothelial 

 lining of vessels. Consistency requires the use of "-derm" for germ layers, leaving 

 " -blast" for formative cells. 



In addition to the trophoderm and ectoderm, the hypothetical stage 

 shown in Fig. 368, A, exhibits a yolk-sac completely lined with entoderm. 

 Between the trophoderm and entoderm, the mesoderm has appeared and 

 is separating into somatic and splanchnic layers, with the body cavity 

 between them. The somatic mesoderm is closely applied to the trophoderm, 

 and together they form the chorion; the splanchnic mesoderm is against 

 the entoderm of the yolk-sac, and forms the outer layer of its wall. The 

 early and rapid development of the mesoderm is characteristic of human 

 embryos, as may be inferred from the later stages. 



In the diagram Fig. 368, B, the amniotic cavity has appeared in the ecto- 

 derm. It is believed to arise as a cleft in a solid mass of cells, and not by 

 the coalescence of ectodermal folds as in the chick; however, in the young- 

 est human embryos observed, it is completely formed. The entoderm 



