WORK IN THE HATCHING-HOUSE. 53 



epidermis, and the nervous stratum into the mucous layer. The 

 parts of the organs of special sense which arise from the epiblast 

 are developed from the nervous layer. In the trout (GSUachar) 

 both layers are continued over the yolk-sac." Certain cells in the 

 median line of the embryo, underneath the medullary groove, give 

 rise to the notochord. The mesoblast very soon forms two lateral 

 plates, one on each side of the body, and between them is placed 

 the notochord. " The general fate of the two mesoblastic plates is 

 the same as in Elasmobranchs. They are at first quite solid, and 

 exhibit relatively late a division into splanchnic and somatic layers, 

 between which is placed the primitive body cavity. The dorsal 

 part of the plates becomes transversely segmented in the region 

 of the trunk, and thus gives rise to the mesoblastic somites, 

 from which the muscle plates and the perichordal part of the 

 vertebral column are developed ; the ventral or outer part remains 

 unsegmented. The cavity of the ventral section becomes the per- 

 manent body cavity. It is continued forward into the head 

 (CEllachar), and part of it becomes separated off from the rest 

 as the pericardial cavity. . . . The hinder end of the embryo, 

 which is placed at the edge of the blastoderm, is somewhat pro- 

 minent, and forms the caudal swelling ; the axis of the embryo is 

 marked by a shallow groove. The body now rapidly elongates, 

 and at the same time becomes considerably narrower, while the 

 groove along the axis becomes shallower and gradually disappears. 

 . . . The trunk grows in the usual way by the addition of 

 fresh somites behind. After the yolk has become completely 

 enveloped by the blastoderm, the tail becomes folded off, and 

 the same process takes place at the front end of the embryo." 

 During the latter stages " the gill-clefts develop from before back- 

 wards. The first cleft is the hyomandibular, and behind this 

 there are the hyobranchial and four branchial clefts. Simultane- 

 ously with the clefts there are developed the branchial arches. 

 The post-oral arches formed are the mandibular, hyoid, and five 

 branchial arches. . . . All these appear before hatching. The 

 first cleft closes up ... about the time of hatching, . . . 

 and about the same time there springs a membraneous fold, the 



