RAIA. 329 



Development. — The skate lays its eggs in the autumn and the 

 young are hatched in early spring. 



The eggs are large yellow spheres which break away from the ovary 

 into the abdominal cavity. Thence they pass into the Fallopian tubes 

 by their internal openings. The male skate is said to thrust the claspers 

 into the cloaca and the base of the oviducts of the female, and to dis- 

 charge sperms down the grooves of the claspers into the oviduct. The 

 sperms then appear to pass up the oviduct and to fertilise the egg 

 in the Fallopian tube. After fertilisation the egg passes down to the 

 oviducal gland in which is secreted an enveloping ego;-case or "purse." 



The eggs contained in these purses are deposited two at a time in 

 moderately deep water, usually amongst dark seaweed. The " purse " 

 is of a tough consistency and a dark greenish-black colour. It is 

 flattened and has long processes at the four corners. The ' ' purse ". has 

 the edges of its two walls at one end lying loosely against each other, 

 allowing free egress but making ingress impossible. In this purse the 

 egg develops slowly, and the young skate on emergence is practically 

 a diminutive adult. During all this period it is sustained by the maternal 

 " yolk," hence the skate has a purely embryonic development and only 

 a lecithal type of nutrition (see page 427). 



Segmentation. — The segmentation is meroblastic, i.e.^ the proto- 

 -plasm is largely aggregated to one pole of the large egg, and there segments 

 or divides into a multicellular disc or cap called the blastoderm. The rest 

 of the protoplasm with few nuclei is scattered throughout the yolk. 

 These nuclei divide and are gradually added to the blastoderm during 

 development. At completion of segmentation the blastoderm has an 

 outer layer or epithelium of cells which represents the epiblast and an 

 inner mass which, with the rest of the egg, forms the hypoblast. 



Gastrulation. — One part of the rim of the blastoderm can soon 

 be distinguished by its greater thickness and is called the embryonic 

 ritn. This represents the future hind end of the embryo, and immedi- 

 ately below it the blastoderm- cells commence to be invaginated, forming 

 an archenteron. Hence this rim is comparable to the dorsal edge of 

 the blastopore in Amphioxus. 



Two separate processes now take place contemporaneously. Firstly, 

 the whole blastoderm commences to envelop the lower yolk-cells by 

 increase of cells at the rim, partly by cells added from the yolk-mass, 

 and partly by division of the blastoderm-cells. This enveloping process 

 does not take place equally all round the edge of the blastoderm or the 

 last point of meeting would be the lower pole, but the embryonic rim 

 does not progress over the yolk ; hence the rest of the rim grows over, 

 and the whole rim gradually closes in immediately behind the blastopore. 



If it be recollected that the edge of the blastoderm is the line of 

 junction of the epiblast and the hypoblast, it is clear that the growth 

 of the former over the yolk-mass is a modified and retarded form of 

 archiblastic invagination, which is called epibolic. 



The process is so slow that at the same time the embryo becomes 

 differentiated in the middle line forwards from the embryonic rim. 

 The nervous system arises along this region as a median dorsal 

 medullary groove which, by the upgrowth and meeting above of its 

 edges or medullary folds ^ becomes convert.ed:-into a complete tube. 



