284 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



d. Latitudinal Plane 



The latitudinal plane of cleavage is similar to the equatorial, but it courses 

 through the cytoplasm on either side of the equatorial plane. For example, 

 the third cleavage planes of the egg of Amphioxus (fig. 1401) and of the frog 

 (figs. 141 E; 142F) are latitudinal planes of cleavage. 



5. Some Fundamental Factors Involved in the Early 

 Cleavage of the Egg 



a. Mechanisms Associated with Mitosis or Cell Division 



There are two mechanisms associated with cleavage or cell division: 



(1) that associated with the chromosomes and the achromatic (amphias- 

 tral) spindle, which results in the equal division of the chromosomes 

 and their distribution to the daughter nuclei, and 



(2) the mechanism which enables the cytoplasm to divide. 



In ordinary cell division or mitosis these two mechanisms are integrated 

 into one process. However, in embryonic development they are not always 

 so integrated. The following examples illustrate this fact: (1) In the early 

 development of insects, the chromatin materials divide without a correspond- 

 ing division of the cytoplasm (fig. 62). (2) During the early cleavage phe- 

 nomena of the elasmobranch fishes, the chromatin material divides before 

 corresponding cleavages of the cytoplasm appear (fig. 158A). (3) In the 

 later cleavage stages of teleost fishes, the peripheral cells of the blastoderm 

 fuse and form a continuous cytoplasm; within this cytoplasm the separate 

 nuclei continue to divide without corresponding cytoplasmic divisions and in 

 this way form the marginal syncytial periblast (fig. 159J, L, M). 



On the other hand, cytoplasmic division may occur without a correspond- 

 ing nuclear division. This behavior has been illustrated in various ways but 

 most emphatically by the work of Harvey ('36, '38, '40, '51) which demon- 

 strates that non-nucleate parts of the egg may divide for a period without 

 the presence of a nucleus. (See, particularly, Harvey, '51, p. 1349.) Simi- 

 larly, in the early development of the hen's egg, a cytoplasmic furrow or 

 division occurs in the formation of the early segmentation cavity without 

 involving a nuclear division (fig. 156C, E). This type of activity on the 

 part of the cytoplasm illustrates the fact that the cytoplasm has a mechanism 

 for cell division independent of the nuclear mechanism. Lewis ('39) empha- 

 sizes the importance of the production of a superficial plasmagel "constriction 

 ring" which constricts the cytoplasm into two parts during ceil division. 



b. Influence of Cytoplasmic Substance and Egg Organization upon 



Cleavage 



1) Yolk. Since the time of Balfour, much consideration has been given to 

 the presence or absence of yolk as a factor controlling the rate and pattern 



