206 Mr. J. T. Cunningham on some 



zation lias occiirrecl or segmentation has commenced. He 

 even goes so far as to include the yolk-segments in pelagic 

 Clupeoid ova, e. g. that of the sprat, in his interpretation. If 

 he followed out this interpretation to its logical consequences 

 he -svould have to maintain that segmentation in the Clupeoids 

 and the sole proceeds to a great extent in the ovary before the 

 &g^ is extruded and before it has been fertilized. 



The capsule which contains the ovicell may be a single 

 continuous layer or may consist of two more or less distinct 

 layers. It has been usually held, in accordance with the 

 definitions adopted by Balfour in his ' Comparative Embryo- 

 logy,' that a vitelline membrane is a primary egg-membrane 

 formed in the ovary by the protoplasm of the ovum, while an 

 egg-membrane formed by the follicular epithelium is to be 

 called a chorion. The term zona radiata is, according to 

 Balfour, to be applied to vitelline membranes which contain 

 numerous radial pores. Thus it is clear that a vitelline 

 membrane may or may not be a zona radiata, but a zona 

 radiata is always a vitelline membrane. Therefore it was 

 scarcely necessary for Prof. Mcintosh to say that in my 

 ' Treatise on the Sole ' I call the zona radiata the vitelline 

 membrane. 



But it must be remembered that it is customary to call the 

 envelope of the Teleosteau egg the vitelline membrane, 

 although it has not yet been proved beyond a doubt that it is 

 farmed by the action of the protoplasm of the ovum. I have 

 shown conclusively that the processes of the egg-capsule in 

 Myxine are formed last in the development of that capsule in 

 the ovary ; the internal part of the capsule is formed first, and 

 it increases in thickness by addition to the outer side. Now 

 the action of the ovum itself could only add new material to 

 the internal surface of the egg-capsule, not to the external 

 surface. Therefore the egg-capsule in Myxine must be formed 

 in part at least by the follicular epithelium. Since many 

 Teleostean ova possess processes of the capsule similar to 

 those of Myxine^ e. g. Belone^ the outer part of the envelope 

 of these ova must also be formed by the follicular epithelium ; 

 for the processes are the last part of the egg-membrane to be 

 developed, not the first. Whether in some ova the Avhole of 

 the egg- membrane is developed by the e^g from without 

 inwards, or whether part is always derived from the egg and 

 part from the follicular epithelium has not yet been proved. 

 Meantime, where, as in the sole's ovum, a single membrane 

 only is discernible, I see no objection to following the example 

 of such writers as Hackel, KolHker, and Waldeyer, and calling 

 it simply the vitelline membrane. The term zona radiata is 



