MUSEUM OF COMPARATIVE ZOOLOGY. 93 



takes place. Moreover, a diminished thickness in the epithelial cells 

 corresponding to the most rapidly increasing part of the membrane 

 (the filaments), not at the close of the period of their formation, but at its 

 beginning, is the reverse of favorable to his " hypothesis. 5 ' 1 And, finally, 

 I would suggest that the zona is everywhere in contact with the sub- 

 stance of the ovum, and that the increased thickness of the membrane 

 at the poles may be due to the accumulation there of a greater pro- 

 portion of the active protoplasm than is found at the surface elsewhere. 

 Perhaps it might be urged against this view that the explanation would 

 be only partial, — that, while it might account for an increased thick- 

 ness of the membrane at the formative pole, it would leave the con- 

 dition at the opposite pole unaccounted for, and therefore could not fully 

 satisfy the needs of the case. 



I have recently sectioned the eggs of a rather poorly preserved speci- 

 men of Myxine australis, with a view to getting additional light on this 

 question. Although the eggs had attained a considerable size, — 22 mm. 

 long by 6 mm. in diameter, — still there was as yet no indication of the 

 filamentous projections; in fact, I could not trace a membrane continu- 

 ously around the egg. At the formative pole there was unquestionably a 

 membrane about 3.5 /x thick ; it was faintly marked like the zona radiata 

 of teleosts, and it presented a deep micropylar infolding, with a cellular 

 epithelial plug. Nevertheless this membrane gradually grew thinner 

 in passing from the formative pole, until it could no longer be recognized. 

 It had about the same extent as the protoplasmic cap. At this stage 

 there was no more accumulation of protoplasm at the opposite pole 

 than at the sides of the egg ; but there also was no more evidence of a 

 zona radiata at this pole than at the sides of the egg. That is all I 

 can offer at present in reply to the possible objection which I have 

 suggested. If it could be shown that the zona is developed at the 

 nutritive pole of the egg without the presence of an accumulation of 

 protoplasm there, and that the granulosa is more highly developed there 

 than on the sides of the egg, I should admit that a strong case would be 

 made against the view I defend. 



1 It is true the author has offered an explanation of this; viz. that the filaments 

 are formed from the cells at the sides of the process, where the epithelium is very 

 thick, and that they are pushed up by the growth at the base. But I should 

 imagine it would be difficult to explain how secretions from lateral cells could do 

 anything more than increase the diameter of the process. 



