1 86 



CHAPTERS ON EVOLUTION. 



But the further stages in development of different animals run 

 parallel beyond the " mulberry stage " of progress. The morula, as 

 we have seen, becomes a " planula " a stage we saw distinctly in the 

 sponge (Fig. 87, 4), and which is repeated with but little variation in 

 the development even of the highest animals. Thus the " planula " 

 appears to be well-nigh as universal in its occurrence as the " morula." 

 But we saw that the planula in due course became the bag-shaped 

 structure named gastrula (Figs. 87, 5 ; 89, 4 ; and 92, 4). The wall of 

 the planula is pushed in upon itself on one side, a central cavity being 

 thus formed, bounded by a double wall, and communicating with the 



outer world by the 

 mouth. Such is the 

 "gastrula"; and in its 

 composition we are 

 able to discern the 

 two primitive layers, 

 named, as we have 

 seen, ectoderm () 

 and endoderm (en), or 



FIG. 101. SEGMENTATION OF FROG s EGG. ... . . \ ,', 



epiblast and hypoblast. 



A third layer, the mesoblast, makes its appearance between these two 

 primary membranes, and from these three layers, as we have already 

 seen, all the structures of the future animal are in due course deve- 

 loped. It seems perfectly certain, then, that if the mulberry stage 

 constitutes a first landmark in the development of the animal kingdom 



at large, no less does the 

 " gastrula-stage " form 

 a second resting-place 

 in the track of life. 

 Since, as Haeckel and 

 other embryologists 

 have shown us (Fig. 

 102), the gastrula stage 

 of development (with 

 its primitive mouth \nt] t 

 body-cavity or stomach 

 [/], and double layers 

 \ec and en~\) occurs 

 equally in the zoophyte 

 (5) and worm (i); is as 

 typical of the star-fish 

 (2) as of the crustacean 

 (3); and aids as ma- 

 terially in the formation of the snail (4) as in the development of 

 the vertebrate (6). After its " gastrula-stage," each animal form may 



6 6 



FIG. 102. GASTRULAS OF VARIOUS ANIMALS. 



