128 EVOLUTION OF LIFE. 
guished from the yellow yolk surrounding it. We have 
tried to explain how, by a continued process of cell-division, 
the contents of the egg of a mammal assume a mulberry- 
shaped form, and the gradual conversion of this mulberry 
into the External and Internal blastodermic membranes. 
If the yolk of the chicken be examined before it is sur- 
rounded by the semi-fluid substance (Fig. 173) and shell, 
there will be found lying on the top of the yellow yolk a 
membrane (Fig. 173, 0) in which may be seen the Germinal 
vesicle and Germinal spot; by a process of cell-division, 
known as partial segmentation, this membrane is trans- 
formed into a heap of balls which gradually assume the 
form of the two membranes which, we have stated, are found 
lying upon the yellow yolk of the freshly-laid egg. In the 
course of development a third membrane appears between 
these two. The partial fusion of these membranes makes 
the Primitive trace, which passes from the oval to the 
guitar-shaped form (Fig. 177). The Middle membrane 
splits into two»layers (Fig. 175), the Upper uniting with 
the External blastodermic membrane, the Lower bend- 
ing down on the Internal blastodermic membrane (Fig. 
176,c). The Middle and Internal blastodermic membranes 
now grow gradually downward around the yellow yolk, 
and finally inclose it. At this stage the embryo chick 
corresponds to Fig. 177, representing the embryo of a 
mammal. We see, therefore, that the development of the 
chick and the mammal is the same, while the difference 
between their eggs is not an essential one,——the nutriment 
for the mammalian egg being furnished from time to time, 
while that for the bird's egg is supplied at once in the form 
of yolk. A homely illustration of this difference is that 
of a man who receives his yearly food. from day to day, 
and of one who receives his yearly food at once. The 
only part of the chicken's egg which corresponds to the 
mammal's egg is the membrane with its Germinal vesicle 
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