560 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



i ; 



B 



FlO. 247. Bl:i-t<Ml-riii of '/'<///,/ with 

 iii.-hill:iry f.il.ls ( x 18). (After Ziegler, 

 1892.) 



.i::il.H. i'.'l I I 



i.,im<l<-il |,i..j.-rti..n n,-;ir th- 



ant.-11'.i edgOfttM l.l:i*t<.lriin i*thi- l.nl^inx 

 :i-ttimi cavity. In (' tin- 



. in. i.-, i. .mi i pioaoni -ii-\a- 



t lie Hurface oi th. blastoderm |.:nall.'l t<> 







period and the young born in an 

 advanced stage [Notidanus, Mus- 

 telus, Galeus, Carcharias, Zygaena, 

 Lamna, Alopias, Cetorhinus, Acan- 

 thias, Scymnus, Squatina, Torpedo, 

 Trygonidae, Myliobatidae]. Amongst 

 the viviparous Elasmobranchs pre- 

 served developmental stages of 

 Torpedo (Fig. 247) may be obtained 

 from Naples, and of Acanthias from 

 various marine laboratories. 



Amongst the oviparous forms 

 certain species of Skate (Eaia) are 

 used as food-fishes and their eggs 

 can frequently be obtained in 

 quantity at trawling centres. In 

 such cases arrangements can be 

 made with local fish-dealers to send 

 on by post the " skate-purses " taken 

 from the oviducts when the fish 

 are cut up. 1 The eggs of the 

 different species differ in size and 

 in the characters of the shell 

 shape, colour, degree of translu- 

 cency (Williamson, 1913). Of the 

 European species R. batis is the 

 most convenient species to use ; 

 the normal period of spawning is 

 from December to April but the 

 retarding effect of the low tempera- 

 ture is so great that December eggs 

 are practically overtaken in their de- 

 velopment by the April eggs. The 

 complete period of development is 

 roughly 20 months, most of the 

 eggs hatching about August. 



The eggs should be posted in 

 damp seaweed. On arrival the 

 soft sticky marginal zone of tin- 

 shell, which separates off except at 

 one end and serves to anelmr 1,1 u> 

 egg to the sea-bottom, is removed, 

 and I lie tlale is marked in ink \\itli 

 a \\oinleii style upon the llat pnrtinn 

 i.f shell hel \veen 1 he t \\ horns. 



OB ol.si-rvv.l out of iiuiny tlmusiii'l.s >f gg8 only mi.- r.i.-r ,|' thr 

 <.f i wo eggs within ;i - OIDIIIUH shell. 



