106 



General Biology 



If the yolk is evenly distributed, the newly formed cells will be more or 

 less of equal size. 



Often the yolk collects at the lower portion of the egg. This is 

 undoubtedly due to the force of gravity. In such cases the protoplasm 

 gathers at the upper end. The upper end is then called the active, 

 formative, or animal pole and the lower the passive, nutritive, or veg- 

 etable pole. The polar bodies are usually freed at the formative pole. 

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

 divide less rapidly than those in the region where there is an excess 

 of protoplasm. In fact the yolk may be so excessive as not to permit 

 any division 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 a moderate amount of yolk accumu- 

 lates 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 

 active 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 assume 

 a superficial position and surround the 

 unsegmented yolk. (Illustrated in the 

 eggs of insects and other arthropods.) 



c J> 



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 blastoccele; bp., 

 blastopore. (After Morgan.) 



BLASTULATION AND 

 GASTRULATION 



As segmentation continues the 



blastomeres remain attached to each 



other and form 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 



