3IATURATI0N OF OVUM IN ECHINUS ESCULENTUlS. 197 



body extrusion. I have seen it in various forms. In fig. 30 

 are seen round the constricting spindle a series of points 

 which afterwards^ as seen in fig. 23^ condense to form a ring 

 round the remains of the spindle. It seems to persist for 

 some time, fig. 33^ and then disappears. 



A final point still remains to be described. When the 

 matured nucleus retires towards the centre of the egg all 

 remains of the reticular or kinoplasmic zone have dis- 

 appeared^ and the nucleus lies surrounded by the alveolar 

 yolk, while round the periphery of the egg the kinoplasmic 

 girdle has narroAved down into a delicate layer of differenti- 

 ated protoplasm. In sublimate material this is seen as a 

 distinct layer, in which large microsomes are arranged 

 regularly side by side. In the osmic-acid material the dis- 

 tinction is less sharp, but there is generally a difference in 

 the characters of the surface layer. I think that possibly 

 this layer has to do with the formation of the membrane 

 thrown off when the selected spermatozoon enters the egg, 

 and, as has been said, I refer it to the kinoplasmic zone 

 which is differentiated on the breaking down of the germinal 

 vesicle. 



History of the Chromatin. — As has been described 

 the greater part of the nuclear reticulum is rejected, and 

 gives I'ise probably to the reticular zone round the trans- 

 formed germinal vesicle. Close to the base of the neck of 

 the invading cytoplasm is found an irregular mass of chro- 

 matin, just as Matthews describes for Asterias, which is pre- 

 sumably the chromatin destined to form the future chromo- 

 somes, figs. 5 — 7. This condensation of chromatin at one point 

 perhaps corresponds to Moore's (1895) synaptic phase, though 

 only a part, not the whole of the chromatin, as in spermato- 

 genesis, is involved in the condensation. Emerging from this 

 condensed mass are seen in figs. 5 and a series of separate 

 elements as to the number of which I am not certain, but I do 

 not think there are more than at a later stage. The following 

 stages, figs. 8 and 9, involve the collection of this mass of 

 chromatin elements into the central plate before described. 



