TYPES OF CLEAVAGE 



1ST. FURROW NEARLY 



MERIDIONAL 



MARGINAL CELLS 



Fig. 152. Cleavage in the egg of the sturgeon, Acipenser sturio. (After Dean, 1895.) 

 (A. B) First and second cleavage planes are approximately meridional. (C) Third 

 cleavage planes are vertical, usually parallel to first cleavage plane. (D) Fourth cleavage 

 planes are vertical, cutting off four central cells from the 12 marginal cells. ( E. F) 

 Later cleavage stages. 



Necturus, although the furrows in the yolk pole area are retarded more and 

 are definitely superficial. The third and fourth sets of cleavage furrows are 

 vertical and succeed in cutting off four central cells from twelve larger mar- 

 ginal cells (fig. 152). Cleavage in this form is more holoblastic in its essential 

 behavior than that in the egg of Amia and Lepisosteus described below 

 (Dean, 1895). 



5) Amia calva. Amia calva is a species of bony ganoid fishes, and it repre- 

 sents one of the oldest living species among the fishes. Its early embryology 

 follows the ganoid habit, namely, its cleavages adhere to the meroblastic 

 pattern of the teleost fishes, with the added feature that the furrows eventually 

 pass distally toward the vegetal pole of the egg. A few yolk nuclei appear to 

 be formed during cleavage. These nuclei aid in dividing the yolk-filled cyto- 

 plasm into distinct cells. The latter gradually are added to the early blastomeres 

 and to the later entoderm cells of the developing embryo. In other words, 

 cleavage in this species is holoblastic, but it represents a transitional condition 

 between meroblastic and holoblastic types of cleavage. 



The egg of Amia assumes an elongated form, averaging 2.2 by 2.8 mm. 

 The germinal disc is. a whitish cap in the freshly laid egg, reaching down over 

 the animal pole to about one third of the distance along the egg's longer axis. 



