6l2 THE STRUCTURE AND DEVELOPMENT 



Internal to this is a thin membrane, the equivalent, according to 

 Eimer, of the membrane found by the same author within the 

 zona in Reptilia. A membrane equivalent to the thick vitelline 

 membrane of Elasmobranchii would seem to be absent in most 

 instances, though a delicate membrane, external to the zona, has 

 not infrequently been described ; Eimer more especially asserts 

 that such a membrane exists in the perch within the peculiar 

 mucous covering of the egg of that fish. 



In Petromyzon, a zona radiata appears to be present 1 , which 

 is divided in the adult into two layers, both of them perforated. 

 The inner of the two perhaps corresponds with the membrane 

 internal to the zona radiata in other types. In Amphibia the 

 single late formed and radiately striated (Waldeyer) membrane 

 would appear to be a zona radiata. If the suggestion on page 

 605 turns out to be correct the ova of Mammalia possess both a 

 vitelline membrane and zona radiata. E. van Beneden 2 has, 

 moreover, shewn that they are also provided at a certain period 

 with a delicate membrane within the zona. 



TJu reticuhtm of the germinal vesicle. In the course of 

 description of the ovary it has been necessary for me to enter 

 with some detail into the structure of the nucleus, and I have 

 had occasion to figure and describe a reticulum identical with 

 that recently described by so many observers. The very interest- 

 ing observations of Dr Klein in the last number of this Journal 3 

 have induced me to say one or two words in defence of some 

 points in my description of the reticulum. Dr Klein says, on 

 page 323, " I have distinctly seen that when nucleoli are present 

 the instances are fewer than is generally supposed ; they are 

 accumulations of the fibrils of the network." I have no doubt 

 that Klein is correct in asserting that nucleoli are fewer than is 

 generally supposed ; and that in many of these instances what 

 are called nucleoli are accumulations, " natural or artificial," of 

 the fibrils of the network ; but I cannot accept the universality 

 of the latter statement, which appears to me most certainly not 

 to hold good in the case of ova, in which nucleoli frequently 

 exist in the absence of the network. 



Again, I find that at the point of intersection of two or more 



1 Carlberla, Zeit. /. wiss. Zoo/. Bd. xxx. a Loc. dt. 



3 [Quarterly Journal Microscopical Science, July 1878.] 



