FROM SIMPLE CELL TO COMPLEX ANIMAL 39 



53. Forms of Segmentation. — The conditions suggested above give rise to the 

 following classes of segmentation. 



A. Total segmentation (in which the planes of cleavage pass through the egg) . 



I. Equal: in which there is little yolk material, and that is well distributed. 

 (Illustrated in most of the lower invertebrates and mammals.) Fig. 

 13.^- 

 II. Unequal: in which there is a moderate amount of yolk which accumulates 

 at the passive pole. The cells at the active pole are more numerous and 

 smaller than at the passive. (Illustrated in many moUusks and in the 

 Amphibia.) Fig. 13, B. 



B. Partial segmentation (in which a portion of the egg does not divide). 



I. Discoidal: in which there is an excessive amount of yolk, with the 

 nucleus and a small mass of protoplasm occupying a disc at the active 

 pole. This disc alone segments, and the embryo lies upon the yolk. 

 (Illustrated in the eggs of fishes, birds and reptiles.) Fig. 13, C. 

 II. Peripheral: in which an excess of yolk collects at the centre of the ovum, 

 with the protoplasm at the periphery. The dividing nuclei assume a 

 superficial position and surround the unsegmented yolk. (Illustrated in 

 the eggs of insects and other arthropods.) Fig. 13, K. 



54. Blastula and Morula. — As cleavage continues the blasto- 

 meres remain associated in a spherical mass. The individual 

 cells project beyond the general surface not unlike the lobes 

 of a mulberry, and for this reason this stage is called the morula 

 or mulberry stage (Fig. 13, 2). By the growth of the cells and 

 by the imbibition of water the morula may become a hollow sphere 

 of cells (blastula) the central cavity of which is filled with fluid. 

 The cavity is termed the segmentation cavity (Fig. 13, s.c). 

 Some animals, such as Volvox, never develop beyond the mor- 

 ula or the blastula stage. 



55. Gastrula. — In those eggs in which the segmentation is 

 total, a next important step is the pushing in of that side of 

 the blastula which corresponds to the original nutritive pole. 

 The process is known as invagination, and the product as a 

 gastrula (Fig. 13,4). It takes place much as one might suppose 

 one side of a hollow rubber ball to be dimpled or infolded by the 

 exhaustion of the air within. The gastrula is to be described 

 as made up essentially of two layers of cells, one external and 

 called ectoderm, and one within called entoderm (Fig. 13, 4). 

 The segmentation cavity may be wholly obliterated ; in that case 

 the entoderm and ectoderm come to lie in contact. The cavity 

 of the invagination of the gastrula is the archenteron or embryonic 



