226 



In the Skate ( JRaia) there are two ovaries ; otherwise the 

 system is similar. See pages 213, 214. 



Write a general Account of the Development of Scyllium. 



The egg is fertilised in the upper part of the oviduct, and 

 is enclosed in a horny quadrangular capsule (formed in the 

 oviducal gland), which has its corners produced like tendrils, 

 for mooring it to seaweeds. Development is slow, and the 

 young dogfish emerges as a miniature adult. 



Segmentation is meroblastic, and restricted to a small 

 germinal area on the greater mass (yolk) of the egg. The 

 resulting blastoderm is a flat disc of cells, the outermost 

 stratum of which is the ectoderm. Between the lower 

 cells is the segmentation cavity, which is later obliterated 

 by the growth of mesoderm. The blastoderm gradually 

 extends over the surface of the yolk mass, and its thickened 

 posterior rim (future hind end of embryo) becomes in- 

 voluted ; the archenteron is thus established, its roof being 

 the inturned cells (endoderm or hypoblast). The further 

 development of the roof is largely by overgrowth (epiboly), 

 and the floor is derived from yolk-epithelium, formed by 

 cells which lie next the yolk. The gastrulation process 

 is completed by delamination. (See Part I., page 15.) 



Forwards, mid-dorsally, from the embryonic rim, a 

 medullary groove appears on the ectoderm. By upgrowth 

 and union of the medullary folds, the groove becomes the 

 neural tube ; it remains open for a time in front, and also 

 behind, until the folds meet and cover in the blastopore, 

 which then becomes the neurenteric canal. (See Amphioxus, 

 page 190.) The notochord is formed, between nerve tube 

 and gut, from the lower cells ; and at each side of it solid 

 gastral mesoderm is developed, which unites with the 

 peripheral mesoderm (formed between the ectoderm and 

 involuted endoderm). 



Gradually the embryo becomes folded off from the yolk, 

 which remains connected with its body by a short con- 

 striction, the yolk stalk. The yolk-sac is the extra -mn- 

 bryonic extension of the blastoderm ; and upon its surface 

 are the vitelline vessels, which absorb the yolk for the 

 nourishment of the embryo. The gill-clefts are formed by 

 involution, and from these grow out delicate filamentous 

 (so-called external) gills, provisional structures Avhich 

 atrophy towards the close of embryonic life. 



For development of principal organs see Craniata, pages 

 194 to 214. 



