SIMPLEST FORMS OF GENERATIVE PROCESS. 441 



different. In the highest Mollusca, however, as also in Fishes, Reptiles, 

 and. Birds, the portion of the yolk which undergoes subdivision is com- 

 paratively small ; and the great mass of the vitellus is destined to be 



Successive stages of segmentation in the vitellus of the Ovum of Ascaris acuminata : A, ovum recently 

 impregnated, the yolk-bag slightly separated from the enveloping membrane; B, first fission into two 

 halves; c, second fission, forming four segments; D, yolk, now divided into numerous segments; E, formation 

 of "mulberry mass" by further segmentation; F, the mass of cells now beginning to show the form of the 

 future worm; G, further progress of its evolution; H, the worm, formed by the conversion of the yolk-cells, 

 now nearly mature. 



subsequently absorbed into the substance of the germ, by a process 

 analogous to that by which the food of the adult is imbibed. Hence 

 the portion of their yolk which undergoes subdivision, and helps to con- 

 stitute the "mulberry mass," may be termed the "germ-yolk," whilst 

 the remainder may be designated as the "food-yolk." 



785. When the whole of the yolk is taken into the mulberry mass, 

 the formation of the embryo is usually the result of the progressive 

 metamorphosis of its parts ; the cells of the surface being converted into 

 the integument, and those of the inner part into the internal organs. 

 This is the case, for example, in the Intestinal Worm, some of the stages 

 in whose development are shown in Fig. 135. The embryonic con- 

 dition of many of the organs is frequently retained, at the time when 

 the young animal comes forth from the egg ; those parts only being 

 completed which are necessary to enable it to obtain its nutriment. 

 Other organs are subsequently evolved, at the expense of the food thus 

 introduced ; and thus a complete change or metamorphosis may take 

 place, in regard alike to external form and to internal structure, between 

 the larval and adult states. Of this phenomenon we have charac- 

 teristic examples in the groups of Insects and Batrachia ; and although 

 it was formerly considered exceptional, it is now known to be the or- 

 dinary occurrence among the lower tribes of animals ; it being com- 

 paratively rare for any of them to come forth from the egg under their 

 adult forms. This change is sometimes obviously gradual, as in the 

 progressive advance of the Tadpole into the condition of the Frog ; but 

 it is sometimes apparently sudden, as when the Chrysalis skin is thrown 

 off, and the perfect Insect comes forth. In the latter case, however, 

 the change is really just as gradual as in the former ; since the develop- 

 ment of the organs characteristic of the perfect Insect is taking place 

 during the whole of the Chrysalis period, to be displayed and brought 



