106 



GENERAL BIOLOGY 



This causes the blastomeres at the nutritive pole to become larger, and 

 divide less rapidly than those in the region where the protoplasm is in 

 excess. In fact the yolk may be so excessive as not to permit any divi- 

 sion at all within it. 



Two forms of segmentation are usually given : 



A. Total segmentation. 



I. Equal: In which there is little yolk material and that 

 well distributed. (Illustrated in most of the lower invertebrates and 

 mammals.) 



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 

 mollusks and in Amphibia.) 



B. Partial segmentation. 



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 ac- 

 tive pole. This disc alone segments 

 and the embryo lies upon the yolk. 

 (Illustrated in the eggs of fishes, birds, 

 and reptiles.) 



II. Peripheral: In which an 

 excess of yolk collects at the center of 

 the ovum, with the protoplasm at the 

 periphery. The dividing nuclei as- 

 sume a superficial position and sur- 

 round the unsegmented yolk. (Illus- 

 trated in the eggs of insects and other 

 arthropods.) 



As segmentation continues the 

 b^astomeres remain attached to each 

 other and from a spherical mass (Fig. 

 35). If the individual cells project out from the mass and the sphere is 

 more or less solid, it resembles a mulberry and is called a morula ( 



), but if it becomes a single layer of cells and is hollow 

 it is known as a blastula ( ). In the latter case the hollow 



portion in the center is filled with a fluid. The hollow space itself is 

 called the segmentation cavity. 



If this blastula indents (just as though one were to take a hollow 

 rubber ball and push in one side with a finger), there are two layers 

 in the indented region. The outer layer is called the ectoderm or 

 epiblast, and the inner the entoderm, endoderm, or hypoblast, while the 

 entire two layered mass is known as a gastrula ( ). 



The indentation is also called invagination and gastrulation (Fig. 36). 

 Having indented, the indented portion draws together to form a 



Fig. 35. 



A, vertical section through a segmenting 

 ovum in the blastula stage. B, C and D, 

 similar sections through later stages. Bl., 

 segmentation cavity or blastocoele ; bp. r 

 blastopore. (After Morgan.) 



