90 



ACIPEXSER. 



most Vertebrata the yolk-cells form a protuberance of the part of the 



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Fig. 56. Diagrammatic longitudinal section thbough the anterior part of 

 the trunk of a larva of acipenser to shew the position occupied by the yolk. 



in. intestine; st. stomach filled with yolk; (es. oesophagus; I. liver; lit. heart; 

 ch. notochord ; np.c. spinal cord. 



alimentary canal, immediately 

 behind the duodenum. The yolk 

 may either, as in the lamprey 

 or frog, form a simple thicken- 

 ing of the alimentary wall in this 

 region, or it may constitute a 

 well-developed yolk- sack as in 

 Elasmobranchii and the Amni- 

 ;ota. In either case the liver is 

 placed in front of the yolk. In 

 ) the Sturgeon on the contrary the 

 "lyolk is placed almost entirely in 



spc. 



pr.n 



front of the liver, and the Stur- 



THROUGH 

 LARVA OF 



Fig. 57. Transverse section 



THE region of THE STOMACH OF J 

 ACIPENSER 5 MM. IN LENGTH. 



St. epithelium of stomach; ?//w yolk; ch. 

 notochord, below which is a subnotochordal 

 rod; in-.n. pronephros; ao. aorta; mj}. muscle- 

 plate formed of large cells, the outer parts of 

 which are differentiated into contractile fibres ; 

 sp.c. spinal cord; he. body cavity. 



Yigeon appears to be also peculiar 

 )(in that the yolk, instead of con- 

 ] stituting an appendage of the ali- 

 I nientary tract, is completely en- 

 closed in a dilated portion of the 

 ., tract which becomes the stomach 

 \(figs. 56 and 57). It dilates this 

 portion to such extent that it 

 might be supposed to form a 

 true external yolk-sack. In the 

 stages before hatching the glan- 

 dular hypoblast, which was es- 

 tablished on the dorsal side of 

 the primitive mesenteron, en- 

 velops the yolk-cells, which fuse 

 together into a yolk-mass, and 

 lose all trace of their original 

 cellular structure. 



