622 MAMMALIAN EMBRYOLOGY 



It will be noted from what has just been said that considerable de- 

 velopment has already taken place by the time the fertilized egg reaches 

 the uterus. Or, to repeat what was said above, it is as a blastodermic 

 vesicle that the mammalian egg reaches the uterus after fertilization. 



At this stage, two important points must be considered : 



First, the method of the formation of germ-layers, and 



Second, the method by which the blastodermic vesicle attaches itself 

 to the uterus of the mother. 



Formation of the Germ-layers. 



The inner cell mass spreads out rapidly so as to form an inner lining 

 to the sub-zonal layer. It is this inner lining which is the entoderm. 



The sub-zonal layer becomes the ectoderm. 



As there is a tremendous variation in the way germ-layers are 

 formed in mammals, it may be well to think of the following example 

 as a help in understanding some of these variations. 



Suppose a group of football players who had already played together 

 in previous years were to come together again. Each would immedi- 

 ately take his place without any preliminary instruction. So, we may 

 think of the embryonic cells in the higher mammals taking a definite 

 place and then developing from there on, rather than passing through 

 all the stages of gastrula formation, and this gastrula then actually in- 

 denting to make two layers. That is, we may think of those embryonic 

 cells which are to develop into ectoderm and mesoderm actually taking 

 the proper position to develop into these structures without first becom- 

 ing a single sheet and then indenting. 



This would mean that the undifferentiated cells which are to become 

 ectoderm would arrange themselves on the outer portion, those which 

 are to become entoderm would arrange themselves more inwardly, and 

 those which are to become mesoderm would take their place between 

 these two layers and then all three could begin developing at about the 

 same time and grow simultaneously. 



In the lower mammals such as the cat, dog, rabbit, etc., this inner 

 cell mass (entoderm) keeps pace with the sub-zonal layer, so that the 

 original cavity which has formed between the jnner cell mass and the 

 sub-zonal layer is now surrounded by an inner layer of entoderm, while 

 the outer layer still remains sub-zonal. In the higher forms such as the 

 primates, that is, in man and the higher apes, the inner cell mass does 

 not grow as rapidly as the sub-zonal layer. There is, therefore, a second 

 cavity formed within the inner cell mass of entodermal cells. 



It is the remaining portion of the inner cell mass, after the entoderm 

 has thus separated from it, which is the ectoderm. The sub-zonal layer 

 is then called the trophoblast (Fig. 356, D). This trophoblast serves 

 as the attachment of the blastodermic vesicle to the walls of the uterus. 



We see from what has been said that a true mammalian gastrula 



