DEVELOPMENT OF FISHES. ' ' 293 



' exclusion.' When the membranes or appendages of the intra-ute- 

 riue embryo contract no adhesion with the parietes of the uterus, the 

 fish is said to be ' ovo-viviparous : ' when such adhesion by inter- 

 lacement of vascular surfaces takes place, the species is said to be 

 * viviparous.' The period of fcetation passed within tlie body of the 

 parent is called 'gestation:' that which takes place in natural ca- 

 vities on the exterior of the body of the parent, or in nests artificially 

 prepared, is called 'incubation;' but these are accidents to fostation, 

 which may go on, as it does in most osseous fishes, independently 

 of either kind of protection. 



Seminafion. — Tlie progress of the testis to maturity, when it 

 attains in all osseous fishes a larger proportional bulk than in any 

 other vertebrate animals, commences by the appearance in its tissue 

 of extremely delicate, closed cells, ' the sperm-sacs.' In these the 

 spermatozoa are developed. * They are discharged, as we have seen, 

 in the lowest organised fishes, by a general rupture of the sacs into 

 the abdomen, and are excluded by the peritoneal canals. In the 

 growing milt of osseous fishes the sperm-sacs also yield to the 

 pressure of their contents, but partially coalesce and form tracts or 

 canals, frequently reticulate in their disposition, as in the Shad, 

 and which ultimately discharge their contents into the efferent pro- 

 longation of the general capsule of the gland. The spermatozoa are, 

 at first, mere nucleated cells ; they acquire in the Loach a pyriform 

 figure, with a minute knob at the apex, from which the filamentary 

 ^ ^ vibratile tail is continued {Jiff. 76. a). Professor "Wag- 



J!a.76. d €> ner estimates the length of the pyriform body at -g-^oth 

 of a line, and that of the tail at ^-'^oth of a line.f Pre- 

 vost and Dumas, and Siebold have described the vi- 

 bratile filamentary appendage in the spermatozoa of 

 other osseous fishes. In the Cyclostomes the body 

 of the spermatozoa is cylindrical ; in Sharks it is long 

 "ioach""'""""'" "^ ^"*"^ spirally twisted ; in Pays they are developed in 

 6. Fasciculus of sper- ijundlcs ( /lo. 76. b), which are arranged in a radiated 



matozoa : Rnia Oxy- \./ i' /' ft 



mf"nifiTd' ^^^^"'^''' disposition in the sperm-sac before its rupture. Dr. 

 Davy I observed many of the spermatozoa grouped 

 together in the vas deferens of the Tliornback. The abundance of 

 these locomotive ciliated cells, and of the minuter granular matter in 

 the fluid in which they float, give to the secretion of the testis or soft 

 roe an opaque milk-white colour. 



Germination. — The ova are developed in the ' stroma ovarii, 

 almost simultaneously in Dermopteri and osseous fishes, more suc- 



* cxxxvui. f cxxi. p. 68. \ cxxxiv. 



u 3 



