EVOLUTION DE ZIP 
130 
embryo be viewed in transverse section (Fig. 175), this 
deepening is seen to be produced through the rising up of 
the External blastodermic membrane (Fig. 175, 4) in two 
heaps, called Laminz Dorsales (Fig. 175, Z), which, grow- 
ing towards each other, finally coalesce, thus converting 
the Primitive groove into a tube (Fig. 176, A). This tube 
is the rudimentary central nervous system. Directly under- 
neath this tube, in the Middle membrane, however, is seen 
a cylindrical rod of cells, the Chorda Dorsalis (Fig. 176, v), 
in which are developed the bodies of the vertebra (segments 
of spine). By looking at Figs. 169 to 172 (Dog or Man), 
we see how, by a continually constructing process, the 
upper portion of the Internal blastodermic membrane (Fig. 
169, 7), with that part of the Middle membrane lying upon 
it, is gradually pinched off from the lower (Fig. 169, a), 
until, finally, only a narrow pedicle connects the two. The 
upper pinched-off portion is the primitive alimentary canal 
(Fig. 170, 7), the lower the umbilical vesicle, or yolk-bag. 
The umbilical vesicle (Figs. 169 to 172, 4), in the course 
of development, passes away, the time of its disappearance 
varying in different animals: thus, in the Trout it is retained 
till the sixtieth day. By referring to Figs. 169, 172, it will 
be seen that the alimentary canal and umbilical vesicle are 
composed of two layers. The inner layer, or the Internal 
blastodermic membrane, develops the epithelium of the 
mucous membrane ; the outer layer, or the.lower half of the 
Middle membrane, makes the wall of the alimentary canal. 
This is a very important distinction, since the lower lungs, 
etc., which first appear as buds sprouting from the aliment- 
ary canal, exhibit the same structure. In the Batrachia 
(Frog), and some Fishes, however, the whole of the Inter- 
nal blastodermic membrane, with that part of the Middle 
membrane lying upon it, is used up in the formation of 
the alimentary canal, which is developed in a different 
manner from that of the dog or man; there is, therefore, no 
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