MUSEUM OF COMPARATIVE ZOOLOGY. 91 



which practically confirms that of Buchholz as to the eversion of the 

 outer membrane. According to Cunningham it serves as the so called 

 suspensory filament by which the deposited eggs are attached. He calls 

 the outer membrane the external zona, and agrees with Owsjannikow 

 that it is traversed by pores which are larger and (according to his 

 figure, about four times) farther apart than those of the internal zona. 

 He makes no mention of the outer membrane differing in any other way 

 from the inner, except that it is somewhat thinner. 



Scharff ('87, '87") regards Beddard's zona radiata 1 in Lepidosiren as 

 a " zonoid " layer, which he also finds present in Trigla gurnard us, where 

 it has a thickness of 25 /x, while the true zona is only about 8 fi thick. 

 " Both layers are striped, i. e. provided with minute radial pores," which 

 are apparently continuous through both layers. The zona is firm, 

 granular, and stains deeply ; the zonoid is semifluid, usually devoid of 

 granules, and stains only slightly. In ripe ova the latter disappears 

 entirely. The zona is formed before the zonoid layer. In Blennius 

 pholis there is no zonoid layer. Scharff " has no doubt that the egg 

 membranes originate from the yolk," although, as far as I can under- 

 stand, he advances no new arguments to prove the fact. 



Henneguy ('88, p. 419) says that, notwithstanding the use of high 

 powers, he has been unable to find in the trout the three layers of 

 Owsjannikow or the denticulations described by Stockman. 



A recent paper by Cunningham ('86 a ; deals with the interesting 

 question of the structure and origin of the egg membrane in Myxine 

 glutinosa. Even when the eggs have become 11 mm. long there is no 

 trace of any membrane between the yolk and the single layer of granu- 

 losa cells, which latter project irregularly into the surface of the former. 

 Sections of an egg 16 mm. in length showed that the granulosa had 

 become several cells deep, though not arranged in regular layers, and 

 that there was beneath this, and in direct contact with the yolk, a thin 

 homogeneous membrane. 



It certainly is an interesting fact, to which Cunningham calls atten- 

 tion, that the epithelium is thicker at the poles than at the equator of 

 the elongated ovum, and that the thickness of the membrane varies 

 [directly] with that of the epithelium. W. Midler also observed the 

 same fact. 



In eggs 20 mm. long the follicular cells are elongated, and form prac- 

 tically a single-cell layer, with the nuclei at the ends nearest the yolk. 



1 Compare Beddard ('86 a ), and the summary of his paper given on pp. 66, 67. 



