GENERAL GROWTH OF THE EMBRYO. 



General development of the Embryo. Attention has 

 already been called to the fact that the embryo first appears as a 

 thickening of the edge of the blastoderm which soon assumes a 

 somewhat shield-like form (fig. 33 A). The hinder end of the 

 embryo, which is placed at the edge of the blastoderm, is some- 

 what prominent, and forms the caudal swelling (ts). The axis 

 of the embryo is marked by a shallow groove. 



The body now rapidly elongates, and at the same time 



fjf 



m so 



au.\> m.b 



FIG. 33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. (After 



His.) 



ts. tail-swelling; au.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment; 

 m.b. mid-brain; cl>. cerebellum; md. medulla oblongata ; m.so. mesoblastic somite. 



becomes considerably narrower, while the groove along the axis 

 becomes shallower and disappears. The anterior, and at first 

 proportionately a very large part, soon becomes distinguished as 

 the cephalic region (fig. 33 B). The medullary cord in this 

 region becomes very early divided into three indistinctly sepa- 

 rated lobes, representing the fore, the mid, and the hind brains : 

 of these the anterior is the smallest. With it are connected the 

 optic vesicles (oc} solid at first which are pushed back into the 

 region of the mid-brain. 



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. The free tail end of 



