DEVELOPMENT OF THE ORGANISM 449 



evenly distributed, they cleave totally but unequally. Telolecithal eggs 

 in some cases, as in that of the frog, are still holoblastic and show total 

 and unequal cleavage; in other cases the excessive amount of yolk causes 

 them to be meroblastic and the cleavage to be discoidal. Controlecithal 

 eggs are also meroblastic, but they cleave superficially. Generally 

 speaking, the lower metazoans and the lower chordates exhibit total and 

 equal cleavage. Many of the higher worms and mollusks, as well as the 

 lamprey and frog among the vertebrates, possess total and unequal 

 cleavage. The arthropods exhibit superficial cleavage, and the fishes 

 and higher vertebrates generally show discoidal cleavage. In the mam- 

 mals, however, pronounced and characteristic modifications in develop- 

 ment occur. In eggs exhibiting total cleavage the blastomeres which 

 are formed remain entirely distinct and each possesses a complete wall. 

 In discoidal cleavage, however, the cells adjacent to the yolk remain 

 for a co^isiderable time without a wall on the side next the yolk. In 

 superficial cleavage the first blastomeres formed have no cell wall next 

 the yolk and even after they have migrated to the surface the cell wall 

 next the yolk remains for some time incomplete. 



488. Blastula. — In embryos produced by total and equal cleavage the 

 first appearance of a blastocoel, or segmentation cavity, is the central 

 cleft between the cells in the eight-cell stage, which is open to the outside 

 by crevices between adjacent cells. As the cells multiply and press 

 upon one another these crevices are closed, the central cleft becomes a 

 cavity entirely surrounded by one or more layers of cells forming the 

 blastoderm, and the embryo is a blastula. Thus there is no morula 

 stage. In telolecithal eggs this segmentation cavity lies toward one side 

 of the embryo and is produced by a splitting apart of the cells which 

 earlier formed a solid morula. 



489. Gastrulation. — In Chap. XXV were also noted certain modifica- 

 tions in the manner of gastrulation. In homolecithal eggs gastrulation 

 takes place by invagination, but in some telolecithal and in centrolecithal 

 eggs it is the result of a splitting apart of the cells, or delamination. 

 In other telolecithal eggs gastrulation is accompanied by overgrowth, or 

 epihole, as a result of which the cells at the animal pole grow around and 

 envelop the vegetal pole of the egg, ultimately hiding it from view. This 

 has been described in the development of the frog's egg (Sec. 400). 



490. Mesoderm Formation. — In the chapter already referred to 

 (Chap. XXV) were also described the formation of the germ layers and 

 the fate of the embryonic cavities. In the sponges and in some coelen- 

 terates the wandering cells form a middle layer, which, for reasons 

 previously stated (Sees. 148 and 183), is not considered to be a mesoderm. 

 In the ctenophores for the first time a distinct mesoderm lying between 

 the ectoderm and entoderm is encountered, although it is composed of 

 few cells. From the flatworms onward, however, the mesoderm makes 



