Ill 



Cleavage 



The group of cleavage cells'- formed by the division of the 

 segmentation nucleus is at first nearly spherical in outline, and 

 situated in the middle of the anterior end of the egg about equi- 

 distant from the cephalic pole and the ventral, dorsal and lateral 

 surfaces. By rapid multiplication and migration of its compo- 

 nent cells, this group elongates in a caudad direction until at 

 Stage II (4-6 hours) it extends some two-thirds of the length of 

 the egg toward the caudal pole. The cleavage cells are now 

 arranged in the form of a one-layered meshwork forming a 

 hollow figure whose outline is long conical or pyriform, the 

 slender pointed end directed caudad, the larger rounded end 

 close to the surface at the anterior pole (Fig. II). A cross section 

 through the anterior portion of an egg at this stage is shown in 

 figure 6. The cleavage cells are here seen arranged in a circle 

 whose position is slightly eccentric with respect to the median 

 axis of the egg, being displaced a trifle toward the ventral 

 (lower) side. The cells are still situated at some distance from 

 one another, as well as from the periphery of the egg. 



With the change in the form of the group of cleavage cells 

 from spherical to elongate there arises a difference between the 

 cells composing its anterior and posterior portions. The differ- 

 ence consists in this : that the cells in the anterior portion of the 

 cleavage figure are more numerous, more compact in form and 



- Blochmann (1887a) has called attention to the fact that the complex 

 made up of the cleavage nuclei, together the protoplasm immediately 

 surrounding them, and the protoplasmic meshwork, including the cortical 

 layer, is a syncitium. It is therefore not strictly correct to use the term 

 "cleavage cells" as applied to the cleavage nuclei and the protoplasm im- 

 mediately surrounding them. The employment of this term may however 

 be justified both on the ground of convenience, and because the cell terri- 

 tory appertaining to each nucleus is always distinguishable, except for a 

 very brief period during the formation of the blastoderm. This position 

 is held by Heider (1889), Heymons (1895), Lecaillon (1897), etc. 



16 



