STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 745 



what has been called by Gotte l " the special sense plate," owing 

 to its being subsequently concerned in the formation of parts of 

 the organs of special sense. We cannot agree with Gotte in 

 regarding it as part of the brain. 



In the keel itself two parts may be distinguished, viz.: a 

 superficial part, best marked in the region of the brain, formed 

 of more or less irregularly arranged polygonal cells, and a deeper 

 part of horizontally placed flatter cells. The upper part is 

 mainly concerned in the formation of the cranial nerves, and of 

 the dorsal roots of the spinal nerves. 



The mesoblast (ms.) in the trunk consists of a pair of inde- 

 pendent plates which are continued forwards into the head, 

 and in the prechordal region of the latter, unite below the 

 medullary keel. 



The mesoblastic plates of the trunk are imperfectly divided 

 into vertebral and lateral regions. Neither longitudinal sections 

 nor surface views shew at this stage any trace of a division of 

 the mesoblast into somites. The mesoblast cells are polygonal, 

 and no indication is as yet present of a division into splanchnic 

 and somatic layers. 



The notochord (nc.) is well established, so that its origin 

 could not be made out. It is, however, much more sharply 

 separated from the mesoblastic plates than from the hypoblast, 

 though the ventral and inner corners of the mesoblastic plates 

 which run in underneath it on either side, are often imperfectly 

 separated from it. It is formed of polygonal cells, of which 

 between 40 and 50 may as a rule be seen in a single section. 

 No sheath is present around it. It has the usual extension in 

 front. 



The hypoblast (/y.) has the form of a membrane, composed of 

 a single row of oval cells, bounding the embryo on the side 

 adjoining the yolk. 



In the region of the caudal swelling the relations of the 

 germinal layers undergo some changes. This region may, from 

 the analogy of other Vertebrates be assumed to constitute the 

 lip of the blastopore. We find accordingly that the layers be- 

 come more or less fused. In the anterior part of the tail 



1 " Ueb. d. Entwick. d. Central Nerven Systems d. Teleoslier," Arc/iiv fur inikr. 

 Anat. Vol. xv. 1878. 



B. . 48 



