74 



GENERAL DEVELOPMENT. 



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 swim- 

 ming 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, in) leading into a large stomodseum which is still without a com- 

 munication with the mesenteron. Ventrally the stomodseum is 

 prolonged for a considerable distance under the anterior part of the 

 mesenteron. Immediately behind the stomodseum is placed the bran- 

 chial region of the mesenteron. Laterally it is produced on each side 

 into seven or perhaps eight branchial pouches (fig. 43, hr.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 em- 

 bryos of Elasmobrauchs, and like them enclosing a central cavity. A 

 similar structure is placed behind the last, and two similar structures 

 in front of the first persistent pouch. This pouch is situated in the 

 same vertical line as the auditory sack (au.v), and would appear there- 

 fore to be the hyo-branchial cleft ; and this identification is confirmed 



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 ; 

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



^ by the fact of two head cavities being present in front of it. At the 

 \ front end of the branchial region of the mesenteron is placed a thickened 

 ^ ridge of tissue, which, on the opening of the passage between the 



stomodseum and the mesenteron, forms a partial septum between the 



two, and is known as the velum (fig. 43, tv). 



<c According to Scott (No. 87) a hyomandibular poucli forming the eighth 

 Kpouch is formed in front of the pouch already defined as the hyobran- 

 \^^chial. It disappears early and does not acquire gill folds \ The tissue 



^ Scott informs me that he has been unable to find the hyomandibular pouch in 

 larvffi larger than 4-8 mm. My material of the stages when it should be present is 



