90 



ACIPENSER. 



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



sp c 



prn 



FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR PART OF 



THE TRUNK OF A LARVA OF AdPENSER TO SHEW THE POSITION OCCUPIED BY THE YOLK. 



in. intestine; st. stomach filled with yolk; <PS. oesophagus; I. liver; lit. heart; 

 ch. notochord; .s^.c. spinal cord. 



alimentary canal, immediately 

 behind the duodenum. The yolk 

 may either-, as in the lamprey 

 or fi-og, 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 

 yolk is placed almost entirely in 

 front of the liver, and the Stur- 

 geon appears to be also peculiar 

 in that the yolk, instead of con- 

 stituting an appendage of the ali- 

 mentary 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. 



FIG. 57. TRANSVERSE SECTION THROUGH 

 THE REGION OF THE STOMACH OF A LARVA OF 

 ACIPENSER 5 MM. IN LENGTH. 



st. epithelium of stomach; yl;. yolk; ch. 

 notochord, below which is a subnotochordal 

 rod; pr.n. prouepbros; ao. aorta; -nip. muscle- 

 plate formed of large cells, the outer parts of 

 which are differentiated into contractile fibres ; 

 ap.c. spinal cord; be. body cavity. 



