96 DIFFERENTIATION III 



Segmentation is irregular, but the swimming blastulae be- 

 come spherical and give rise to normal plutei, at the normal rate. 



Though the male nucleus unites without fusing with the 

 female nucleus, in the spindle developed between the two 

 sperm-centrosomes it does not break up into chromosomes, 

 and eventually passes undivided to one pole and into one 

 cell. The female chromosomes divide in the usual way. The 

 sperm-nucleus degenerates ultimately. It may not even have 

 approached the female nucleus in the earlier stages, but have 

 remained apart from the beginning. 



The same author has found that membrane formation can 

 be incited in the Sea-urchin (Echinus) egg by spermatozoa 

 of a number of Molluscs (Lithodomus, Mactra, Modiolaria, 

 Pecten, Venus, Patella, Gibbula, Murex, Nassa, Trochus, 

 Fusus) and by those of the Polychaets Aricia and Audoui- 

 nia. Only when Mytilus, Mactra, Patella, Aricia, and 

 Audouinia are used does development follow. It has been 

 shown elsewhere that membrane formation and the incite- 

 ment to develop are two independent phases of fertilization. 



For ensuring fertilization with Audouinia sperm treatment 

 with alkali is unnecessary. The sperm enters, the head swells 

 up, and a centrosome and aster are developed. The male 

 nucleus may then unite more or less completely with the 

 female nucleus, or remain apart. In the latter case it passes 

 into one blastomere and degenerates, as does the Mytilus 

 sperm in the Strongylocentrotus egg. In the former case 

 when the spindle is developed and the female chromosomes 

 are placed upon it, the sperm chromatin is seen in the form 

 not of chromosomes but of one or more irregular lumps lying 

 outside, on, or inside, the spindle. 



When in the anaphase the female chromosomes become 

 vesicular, the male lumps of chromatin do the same; they 

 are distributed sometimes to both blastomeres, sometimes to 

 one only. In the blastomeres the sperm chromatin may unite 

 with the (female) nuclei but need not do so, and there is 

 always a tendency for it to become gradually eliminated, for 

 it is found in fewer and fewer cells as development proceeds, 

 and the nuclei are found to be of half the size (surface) of the 



