DEVELOPMENT OF THE SKATE. 425 



containing an ovum. The ripe ova burst into the body- 

 cavity, and enter the single aperture of the oviducts, which 

 are united anteriorly. In the upper part of the oviduct the 

 ovum is fertilised, in the middle part it is enclosed in a 

 " mermaid's purse," secreted from a dilatation known as the 

 oviducal gland. The lower or uterine portions of the ovi- 

 ducts open into the cloaca. 



Development. — The ripe ovum which bursts from the 

 ovary and passes into the oviduct is a large sphere of yolk, 

 with the formative protoplasm concentrated at one pole. It 

 seems that the nuclear division, usually associated with 

 maturation, takes place at a very early stage. In the upper 

 part of the oviduct the ovum is fertilised. As it descends 

 it is surrounded by albuminous material, and by the four- 

 cornered " mermaid's purse " secreted by the walls of the 

 oviducal gland. This purse is composed of a horny sub- 

 stance allied to that of hair and hoof Its corners are 

 produced into long elastic tendrils, which may twine round 

 sea-weed, and thus moor the egg. Rocked by the waves, 

 the embryo develops, and the young skate leaves the purse 

 at one end. 



The segmentation is meroblastic, being confined to the 

 disc of formative protoplasm. From the edge of the 

 blastoderm, which results from the segmentation, some 

 nuclei are formed in the outer part of the subjacent yolk. 

 These yolk-nuclei afterwards share in the making of the 

 embryo. 



At the close of segmentation the blastoderm is a lens- 

 shaped disc with two strata of cells. It is thicker at one end 

 — where the embryo begins to be formed. Towards the 

 other end, between the blastoderm and the yolk, lies a seg- 

 mentation cavity. 



At the embryonic end, the outer layer or epiblast 

 undergoes a slight invagination, beginning to form the 

 roof of the future gut, in other words establishing the 

 hypoblast. This inflected arc of the blastoderm corres- 

 ponds to the blastopore or mouth of the gastrula, which 

 is much disguised by the presence of a large quantity of 

 yolk. As the invagination proceeds, the segmentation-cavity 

 is obliterated. The floor of the gut is formed from the 

 yolk-nuclei. 



