DEVELOPMENT OF ELASMOBRANCH FISHES. 131 



which distinctly belon^^s to the splanchnopleure (PI. XI. fig. 14a). 

 Some are situated external to the segmental involutions ; and 

 others again, though this is not common, in a part of the 

 mesoblast which distinctly belongs to the body-wall (PL xi. 

 fig. 14 6). 



The portion of mesentery in which the primitive ova are 

 most densely aggregated, corresponds to the future position of 

 the genital ridge, but the other positions occupied by ova 

 are quite outside this. Some ova are in fact situated on the 

 outside of the segmental duct and segmented tubes, and must 

 therefore effect a considerable migration before reaching their 

 final positions in the genital ridge on the inner side of the 

 segmental duct (PL xi. fig. 14 6). 



The condition of the tissue in which the ova appear may at 

 once be gathered from an examination of the figures given. 

 It consists of an irregular epithelium of cells partly belonging 

 to the somatopleure and partly to the splanchnopleure, but 

 passing uninterruptedly from one layer to the other. The 

 cells which compose it are irregular in shape, but frequently 

 columnar (PL xi. fig. 14 a and 14 6). 



They are formed of a nucleus which stains deeply, invested 

 by a very delicate layer of protoplasm. At the junction of somato- 

 pleure and splanchnopleure they are more rounded than else- 

 where. Yery few loose connective-tissue cells are present. The 

 cells just described vary from "008 Mm. to "01 Mm. in diameter. 



The primitive ova are situated amongst them and stand out 

 w^ith extraordinary clearness, to which justice is hardly done in 

 my figures. 



The normal full-sized ova exhibit the following structure. 

 They consist of a mass of somewhat granular protoplasm of 

 irregular, but more or less rounded, form. Their size varies 

 from '016 — "OoG Mm. In their interior a nucleus is present, 

 which varies from "012 — '016 Mm., but its size as a rule bears no 

 relation to the size of the containing celL 



This is illustrated by the subjoined list of measurements. 



The numbers given refer to degrees on my micrometer scale. 



Since it is the ratio alone which it is necessary to call 

 attention to, the numbers are not reduced to decimals of a 

 millimeter. Each degree of my scale is equal, however, with 

 the object glass employed, to '002 Mm. 



