CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



Fig. 23. Egg-capsule of Rhinochimana pacifica. Misalci. 

 (Ventral aspect.) Natural size. 



however, may best be considered subse- 

 quently in correlation with similar facts. 



The capsules may also be referred to 

 at the present time in the evidence they 

 present regarding the factors of evolution ; 

 for it is clear that such highly specialized 

 capsules provide a valuable check upon 

 the evolutional process from the standpoint 

 of the obvious "prevision" which they 

 demonstrate. The capsule is, in short, 

 adapted not so much to the egg as to 

 the young fish which it will later contain. 

 Thus it is specialized in accord with the 

 shape of the young fish, its position, and 

 its late physiological needs, ail to a de- 

 gree which is, indeed, probably unequaled 

 in the secondary embryonic membranes of 

 other animals. * This degree of special- 

 ization becomes clearer, moreover, when 

 we take into consideration the formation 

 of the capsule. 



FORMATION OF THE CAPSULE. 



At the time the egg is about to leave 

 the ovary the oviduct is flaccid and is 

 richly suffused with blood ; in fact, from 

 this time onward the oviducal sinusf in 

 which they lie is dilated (plate i, fig. 4, 

 and plate n, fig. 5, ovd. s.\ forming a 



*Cf. Dean, 1904, Biol. Bulletin, vol. VH, pp. 105-112. 



t These sinuses arise in the mesovaria, the walls of which 

 do not become apposed. They are thus longitudinal sacs of 

 blood in which the oviducts lie more or less freely, depend- 

 ing upon the degree of development of the egg-capsule 

 (cf. plate ii, fig. 5, and plate i, fig. 4, left oviduct). In the 

 former figure, however, this condition is not seen favorably, 

 since the oviduct is purposely pushed against the wall 

 of its sinus, thus dislodging the opaque blood, so that the 

 structures of the oviduct can be better described. In 

 the latest stage in the formation of the capsule, on the other 

 hand, the sinus is so filled with the enlarged oviduct that in 

 ventral view it can hardly be seen ; thus in the figure the 

 oviducts appear to lie freely in the body-cavity. The blood 

 supply in the sinus, it may be remarked, is maintained by 

 direct communication with the cardinal (not to complicate 

 the problem as to the relations with the renal portal ) blood- 

 cavities. Between the lines where the mesovarial folds are 

 attached to the dorsal body wall a row of ostia is present 

 (pi. i, fig. 4, o). This method of increasing enormously the 

 oviducts' blood (venous) supply is evidently correlated with 

 the rapid formation of the highly complicated egg-capsule. 

 It can hardly be regarded as evidence of a primitive gon- 

 adial sinus, and we are led to conclude that morphologically 

 the veins of the mesovarium have coalesced, leaving ostia 

 as vestiges of the gonadial veins, e. g., of sharks. 



