366 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



modification of (1) brought about by a hurrying on of the develop- 

 ment of the corpuscles. 



(3) In still other cases the cells of the mesenchyme reticulum in 

 certain localities e.g. in the spaces between the tubules of the 

 pronephros draw in their processes, round themselves off and are 

 carried away in the blood stream as primitive corpuscles. 



This last mode of origin may account for the fact that the 



Fig. 176. — Portions of transverse sections through Elasmobranch embryos illustrating 

 the origin of the heart. 



A, Torpedo, stage with one gill cleft ; B, Torpedo, stage with two gill clefts ; 0, Pristiurus, stage 

 with twenty-five segments ; D, Pristiuriis, stage with forty segments. (After figures by Rnckert 

 (1S88) and Mollier (1906).) d.mc, dorsal mesocardinm ; enc, endocardium ; end, endoderm of foregut ; 

 vie, myocardium ; mes, mesoderm ; N, notochord ; pc, pericardiac cavity ; s.c, spinal cord ; splc, 

 splanchnocoele. 



corpuscles have been observed to make their appearance suddenly in 

 large numbers in the circulating blood. The sudden setting free 

 of large numbers of corpuscles is possibly due to an epidemic of 

 mitosis in the mesenchyme cells, as it is well known that the onset 

 of the mitotic process frequently induces a retraction of the processes 

 of the cell-body and its assumption of an approximately spherical 

 shape. 



It is not proposed to trace out in this volume the further develop- 

 ment of the blood corpuscles — how the originally similar indifferent 



