TYPES OF CLEAVAGE 



311 



BLASTOCOELIC 

 SPACE 



YOLK 



4 th. ,: 



CLEAVAU E ""^^•i'Sii;^-^'"' 



CENTRAL 

 CELL 



'5^ YOLK BED 



Fig. 154. Early development of Lepisosteus osseus. (After Dean, 1895.) (A) Un- 

 cleaved egg, showing germinal disc. (B) First cleavage is trench-like, extending beyond 

 (i.e., laterally) to the margin of the germinal disc. (C) Transverse section of cleavage 

 furrows shown in (B). (D) Four-cell stage. (E) Third cleavage planes are vertical 

 as indicated. (F) Fourth cleavage planes also are vertical. (G) Germinal disc, sec 

 tioned 25 hours after fertilization. Blastocoelic spaces dispersed. 



zontal. While the latter is in progress the fifth cleavages, which are vertical, 

 begin. As a result of the fourth and fifth sets of cleavages, a mass of eight 

 central cells and twenty or more marginal cells arises. Horizontal (i.e., lati- 

 tudinal) cleavages begin among the central cells at this time, and other cells 

 (see cell A, fig. 153F) appear to be budded off from the yolk floor from this 

 period on. The latter are contributed to the growing disc of cells above. 



Four types of cleavage furrows now appear in the growing blastoderm as 

 follows: 



(1) cleavage among the central cells, increasing their number, 



(2) cleavage among the marginal cells, contributing cells to the central 

 cells, 



