NATURAL HISTORi" OF VERTEBRATES. 



walls of the oviduct ns the egg escapes to the outside ; the lower part of that tube 

 may also provide a protective shell for the egg, the form, texture, and chemical nature 

 of which present wide differences in the various groups of oviparous vertebrates. 



Eggs which are free of food-yolk, like those of most mammals, divide completely 

 into two equal parts after fertilization, and this division goes on regularly till a mass 

 of small cells results, which eventually arrange themselves into the three embryonic 

 layers. Such eggs are holoblastic ; but the presence of a large amount of food-yolk in 

 the egg brings about various complications; it prevents the eggs dividing completely, 

 and consequently the process of segmentation is limited to one pole of the egg, and 

 is on this account styled meroblastic. Other processes, too, ai'e rendered more complex 

 by the presence of the mass of non-segmenting food material, so, as an introduction to 

 the jihenomena of development, we shall select the eggs of the toad, which are com- 

 paratively free from food material, and which divide completely, though unequally. 

 The eggs of most mammals are so free from food-yolk that they divide completely and 

 equally, but as the mammals are, probably, descendants of forms with large eggs, the loss 

 of the food-yolk is to be considered as secondary, and as consequent upon the develop- 

 ment of certain other arrangements (the placenta) for nourishing the embryo, which 

 have rendered the other food material unnecessary. But the mammalian eggs have 

 inherited from these earlier types certain complications which render it easier for the 

 beginner to understand the processes of development in the eggs of frogs and similar 

 forms. 



As is well known, the spawn of toads and frogs is composed of groups of eggs sur- 

 rounded by a transparent jelly-like mass, which, like the white of the 

 l)ird's egg, is secreted by the walls of the oviducts, but, unlike that, is 

 not used as food by the developing embryos. The egg itself measures 

 about i^s of an inch in diameter, and that pole which contains the nucleus 

 or germinal vesicle is always more jjigmented than the other. After 

 Fig. 1. — Egg of fertilization the etrsc becomes completely divided into two equal parts, 



Bombinator after , °° . . . ,, i- , ■ , , „ 



second segmen- and very soon thereaiter into tour cells, two oi which are much smaller 

 than the others (Fig. 1). Even at this stage the cells begin to retract 

 from each other, so as to leave a small space between them, which eventually increases 

 as the division of the cell goes on, and becomes the 

 segmentation cavity. The larger cells, which contain 

 the food-yolk, segment more slowly than do those at 

 the other pole of the egg, so that, before the next 

 important stage in development takes place, a spheri- 

 cal multicellular body results, the cells at one pole 

 of which are very much smaller than those at the 

 other pole. At one spot on the line of junction of 

 the small and large cells (Fig. 2, a-), the small cells are 

 turned in, or invaginated, so as to form the lining of 

 a new cavity, the beginning of the intestine, and 

 simultaneously, while the rest of the small cells gradu- 

 tudinai section ally grow over the large ones so as to occupy also the 

 '' ^^vHiyl^i", other pole of the egg, the lining of the intestine be- 

 comes separated from the outer layer by a new middle 

 layer, which grows round from the li])S of the orifice of invagination (Fig. 2, w?, m') 

 In this way the three layers of the embryo are formed, to whicli all the tissues and 



Fig. 2.— Diif,! 

 of en\br>o t 

 m, mesoblast s, se^meutitiou 

 edge of invagination. 



