332 



CLEAVAGE (SEGMENTATION) AND BLASTULATION 



POSIirON OF 2ND. MATURATION SPINDLE 

 POINT OF SPERM PENETRATION 



D. 



CLEAVAGE FURROW OF FIRST CLEAVAGE 



ON PART OF NUCLEUS WHICH ENTERED " BRIDGE' 



Fig. 165. Drawings of cleavage of a partially constricted egg of Triturus viridescens, 

 illustrating delayed nucleation. (Slightly modified from Fankhauser, '48.) (A) Shows 

 constricting loop, point of sperm entrance, and second maturation spindle. The constricted 

 portion to the right will contain the fusion nucleus. (B) First cleavage furrow in right 

 half of egg. (C) Second cleavage. The nucleus in the "bridge" area has migrated into 

 the "bridge." (D) Third cleavage. The nucleus in the bridge area has divided and pro- 

 duced cleavage furrow through the bridge cytoplasm as indicated. One of the daughter 

 nuclei of this cleavage is now in the constricted part of the egg at the left. (E) Fourth 

 cleavage = first division of left half. (F) Blastular stage — late blastula at right, middle 

 blastula at left. 



ectoderm grows over the half of the embryo which failed to develop. Also, 

 the notochord rounds up into a normally shaped notochord but is only half 

 the normal size. Essentially, however, these separated blastomeres develop 

 into "half embryos in which some cells have grown over from the uninjured 

 to the injured side, but in which absolutely no change has taken place in the 

 potency of the individual cells or of the different ooplasmic substances" 

 (Conklin, '06). Similarly, at the four-cell stage isolation of anterior and poste- 

 rior blastomeres gives origin to anterior and posterior half embryos respectively. 



The developing sea-urchin egg has been used extensively for experimental 

 work in the study of isolated blastomeres. In figures 163 and 166A-D are 

 shown the different developmental possibilities which arise from isolated blas- 

 tomeres of the early cleavage stages. Also, in cell-lineage studies on the de- 

 veloping egg of Ascaris, a difference in the developmental potencies of the 

 blastomeres is evident (fig. 164). (See also fig. 145A-D in respect to the 

 early development of the pig.) 



The foregoing experiments and observations and others of a similar nature 

 suggest that, during the early cleavage stages of many different animal species, 

 a sorting-out process is at work which segregates into different blastomeres 



