GENERAL DEVELOPMENT. 



dilatation of the anterior part of the body, and the branchial 

 pouches grow out from the throat. The anus becomes formed, 

 and a neurenteric canal is also established (Scott). The nature 

 of these and other changes will best be understood by a 

 description of the structure of the just-hatched larva. The 

 general appearance of the larva immediately after hatching 

 is shewn in fig. 41, D. The body is somewhat curved ; the 

 posterior extremity being much dilated with yolk, while the 

 anterior is very thin. All the cells still contain yolk particles, 

 which render the embryo very opaque. The larva only exhibits 

 slow movements, and is not capable of swimming about. 



The structure of the head is shewn in figs. 42 and 43. Fig. 

 42 is a section through a very young larva, while fig. 43 is taken 

 from a larva three days after hatching, and shews the parts with 

 considerably greater detail. 



On the ventral side of the head is placed the oral opening 

 (fig. 43, m) leading into a large stomodaeum which is still with- 

 out a communication with the mesenteron. Ventrally the sto- 

 modaeum is prolonged for a considerable distance under the 

 anterior part of the mesenteron. Immediately behind the sto- 



0' V. 



FIG. 42. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA OF 



PETROMYZON. (From Gegenbaur ; after Calberla.) 



o. mouth ; o '. olfactory pit ; v. septum between stomodaeum and mesenteron ; 

 h. thyroid involution ; n. spinal cord ; ch. notochord ; c. heart ; a. auditory vesicle. 



modseum is placed the branchial region of the mesenteron. 

 Laterally it is produced on each side into seven or perhaps eight 

 branchial pouches (fig. 43, br.c], which extend outwards nearly 

 to the skin but are not yet open. Between the successive 

 pouches are placed mesoblastic segments, of the same nature 

 and structure as the walls of the head cavities in the embryos of 

 Elasmobranchs, and like them enclosing a central cavity. A 



