802 TEXTBOOK OF ZOOLOGY 



six types of cleavage, as given below. In general, according to the 

 amount of yolk present, eggs are spoken of as of three kinds. Iso- 

 lecithal or homolecitJial eggs are those in which only a small amount 

 of yolk is present and that uniformly distributed throughout the 

 cytoplasm. Telolecithal eggs are those in which the vegetative half 

 of the egg contains most of the yolk (the vegetative pole is opposite 

 the animal pole, where the polar bodies are given off). In some telo- 

 lecithal eggs the amount of yolk is not very large, and it is in the 

 form of distinct globules; in others, it is concentrated into a dense 

 mass in the form of a large yolk spherule, which is not penetrated by 

 the cytoplasm and therefore cannot be divided by it when the ceU 

 undergoes cleavage. The third characteristic mode of division is that 

 in centrolecithal eggs in which the entire egg is well filled with yolk 

 at the beginning of cleavage, with a nucleus surrounded by cytoplasm 

 forming an island in the center of the egg. Successive divisions 

 carry the plasma islands to the surface and leave the yolk as a cen- 

 trally located mass. 



Cleavage is therefore divided first of all into two kinds, total and 

 partial, depending on whether or not the cytoplasm is able to divide 

 the yolk completely. Eggs having total cleavage are spoken of as 

 holoblastic; those having partial cleavage, as merohlastic. In each of 

 these subdivisions the cleavage patterns take different forms, and 

 therefore allow us to recognize six distinct modes by which the sepa- 

 ration of egg substances into definite areas is accomplished; thus the 

 different plans of embryonic organization are established. These six 

 types are radial, disynmietrical, bilateral, and spiral, which occur in 

 holoblastic eggs ; and superficial and discoidal, in meroblastic eggs. 



Radial cleavage (Fig. 415) occurs in eggs which are not heavily 

 yolk-laden, and hence is marked by a high degree of regularity and 

 symmetry. Eggs of the echinoderms are commonly chosen to illustrate 

 this type, which also occurs in the Porifera and Coelenterata. The 

 first cleavage plane cuts the egg into half from the animal to the 

 vegetal pole, and may pass through any diameter of the egg. The sec- 

 ond cleavage plane again cuts through the poles at right angles to the 

 first. The third cuts the egg somewhere near the equatorial region; it is 

 therefore best to speak of it as latitudinal. The sixteen-cell stage is 

 reached by a meridional cleavage, and meridional and latitudinal 

 cleavages alternate until there is developed a well-formed blastula, the 

 coeloblastula (see below). 



