560 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



Fig. 247. — Blastoderm of Torpedo with 

 medullary folds ( x 18). (After Ziegler, 

 1892.) 



A, stage four (Scammon, 1911) ; B, stage six ; 

 C, stage ten. The rounded projection near the 

 anterior edge of the blastoderm is the bulging 

 roof of the segmentation cavity. In C the 

 blood-islands form a row of conspicuous eleva- 

 tions of the surface of the blastoderm parallel to 

 its edge. 



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 {Haiti) are 

 used as food-fishes and their eggs 

 can frequently be obtained in 

 quantity at trawliug 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 

 Sin the characters of the shell — 

 shape, colour, degree of translu- 

 vicency (Williamson, 1913). Of the 

 European species B. batis is the 

 I 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 the 

 shell, which separates off except at 

 one end and serves to anchor the 

 egg to the sea-bottom, is removed, 

 and the date is marked in ink with 

 a wooden style upon the fiat portion 

 of sbell between the two horns. 



1 Jamieson observed out of many thousands of eggs only one case of the inclusion 

 of two eggs within a common shell. 



