THE FROG. 169 



c. The mesobiast; forming a denser mass {nteso- 

 blastic somite) on either side of the neural tube 

 and notochord; below that point subdivided 

 into two layers — an outer one {somatic layer) 

 which applies itself to the epiblast, and an inner 

 one {splanchnic layer) which invests the hypo- 

 blast and lower layer cells. 



^. The body cavity; the interspace between the 

 somatic and splanchnic mesobiast; more or 

 less obvious, in accordance with the degree of 

 shrinkage of the preparation. 



t). The leading blood-vessels, as under. The dorsal 

 aorta^ median and cylindroidal underlying the 

 sub-notochordal rod; the posterior cardinal 

 veins, large vessels, ovoidal or irregular in 

 section, right and left of the aorta. Blood 

 corpuscles will in all probability be seen within 

 them. 



Examine under a high power. The above 

 vessels will be seen to be formed as excava- 

 tions of the mesobiast, 'each bounded by a 

 single layer of modified cells. 



B. The pro-renal {segmental) duct ; a conspicuous 

 thick-walled tube seen, on either side, lying 

 within the somatic mesobiast immediately be- 

 neath the posterior cardinal vein. 



The larva at 12 — 14 days (cf. i. //). Median lon- 

 gitudinal vertical, to pass through the blastopore 

 (anus). Examine under a low power. 



tt. The body; elongated and enclosing two 

 cavities: — a ventral or archenteric canal, in 

 communication with the exterior at the blasto- 



