94 ANIMAL BIOLOGY. [Parti. 



frog) the persistent blastopore. The lungs, and the glands of the 

 alimentary canal arise as hollow outgrowths therefrom. The 

 heart, and all the blood-vessels arise as tubes within the meso- 

 blast, from which layer are also formed all the muscles, cartilages, 

 and bones of 'the body. The hypoblast gives rise to the epithelial 

 layer of the alimentary cavity and of the glands connected there- 

 with. The epiblast forms the epidermic layer of the skin ; but 

 from it are also formed the central nervous system and certain 

 important structures connected with the organs of special sen- 

 sation. 



We have thus seen how the three germinal layers, the ali- 

 mentary canal with its mouth and anus, the body-cavity, the 

 central nervous system, and the notochord may be formed in 

 the case of an ideal, but not wholly imaginary, animal. The 

 student is clearly to understand that both Fig. 32 and this descrip- 

 tion of the mode of formation of the leading structures in the body 

 are, so to speak, diagrammatic, and introduced merely to aid him 

 in comprehending how these structures are formed in the frog, 

 the fowl, and the rabbit, to which we may now turn. 



Segmentation of the Ovum. (1.) In the frog the mode of seg- 

 mentation does not differ very markedly from that described. The 

 segmentation is complete (holoblastic), that is to say, the planes 

 of division run right through the whole thickness of the ovum, 

 all its substance being segmented or broken up into cells ; but 

 owing to the inequality of the distribution of the food-yolk it is 

 unequal or irregular. Where the protoplasm is most concen- 

 trated, namely, at the animal pole, segmentation is more rapid. 

 Thus the first two furrows form rapidly in the upper half of the 

 ovum, but more slowly extend to the lower half. The first 

 horizontal furrow is not equatorial, but near the animal pole 

 (Fig. 33). And throughout segmentation cell-division goes on 

 more rapidly in the animal hemisphere, so that the cells are 

 smaller there than in the opposite hemisphere, It is usually 

 found that wherever large quantities of food-yolk occur, segmen- 

 tation is more or less unequal, the velocity of segmentation being 

 directly proportional, and the size of the cells inversely pro- 



