INTRODUCTION. 5 



division of the ovum into four parts, and by a similar process are 

 formed eight, then sixteen, then thirty-two spheres, and so on, the 

 spheres becoming smaller and smaller, and each being provided 

 with a nucleus. In short, out of the original ovum, corresponding 



Fio. 3. DIAGRAM OP A SBGMENTRD MEIIOBLASTIO OVUM. 

 Bin, blastoderm ; T>n, yolk. 



to a single cell, a mass of cells is formed, which represents the 

 building-material of the animal body, and which, from its likeness 

 in appearance to a mulberry, is spoken of as a morula. 1 



.-FH 



;. 4. BLASTOSPHEUK. 

 7?/>, blastoderm : FH, segmentation cavity. 



In the interior of this morula a cavity (segmentation cavity) 

 filled with fluid is formed, and the morula is now spoken of as the 

 blastosphere or blastula. The peripheral cells enclosing this 

 cavity form the germinal membrane or blastoderm (Fig. 4, 



1 A segmentation of the entire ovum occurs in Amphioxns, Cyclostomes, Stur- 

 geon, Lepidosteus, Amphibians, and Mammals (with the exception of Monotremes). In 

 Elasmobranchs, Teleosteans, Reptiles, Birds, and Monotremes, in which a very large 

 amount of food -yolk (deuteroplasm) is present in the protoplasm, the egg under- 

 goes only a partial segmentation, the main mass of the yolk remaining undivided 

 and serving merely as nutritive material for the developing embryo. The former 

 are spoken of as holoblastic (Figs. 1 and 2), the latter as meroblastic (Fig. 3). 

 The eggs of the Sturgeon, Lepidosteus, and in a less degree those of Cyclostomes and 

 Amphibians, though holoblastic, approach the meroblastic type. 



