MARINE TELEOSTEAN DEVELOPMENT. 89 



especially the veins have largely increased, and the portal 

 system is indicated. The proximity of the yolk to the great 

 vessels returning to the heart makes it probable that these are 

 connected with its absorption. As the absorption of the yolk 

 takes place, however, before any circulation occurs, it is possible 

 that the open end of the heart takes in the yolk-materials, 

 as Mr H. C. Williamson has recently more clearly pointed out. 



The origin of the blood-corpuscles is still open to discussion, 

 more than one mode of origin being suggested. Briefly the 

 formative elements of the blood seem for the most part to 

 be derived from the periblast, the primary corpuscles being 

 moulded from the detached cells of the sub-notochordal trunks. 



Renal organs. 



Soon after the splitting of the mesoblast into an outer layer 

 (somatopleure) and an inner layer (splanchnopleure) a rod of 

 cells is budded off from the latter. In the transparent pelagic 

 forms, e.g. the cod, we find that a solid cylinder appears on the 

 outer margin of the intermediate cell-mass, and it rapidly 

 develops forward to the pectoral fins, but advances posteriorly 

 more slowly. A lumen is formed by a radial arrangement 

 of its cells. Thus a pair of simple ducts, whose walls consist of 

 a single layer of columnar cells, originate from the pectoral 

 region to the root of the tail. Anteriorly each tube, widely 

 separated from its neighbour, is folded on itself inwards towards 

 the notochord, and ends in a trumpet-shaped infundibular 

 opening. Posteriorly the tubes approach each other and co- 

 alesce into a single tube, which at first is of small capacity 

 with thick walls. Somewhat superficial at first, the ducts lie 

 later ventro-laterally to the notochord and ultimately protrude 

 into the peritoneal cavity. A connection is established pos- 

 teriorly, at the urinary vesicle, with the cavity of the hind-gut. 

 The crozier-shaped loop at the anterior end has in front of it a 

 mass of trabecular tissue in which tubules appear to some 

 extent to enter, but it is also penetrated by the basilar plate of 

 the skull. Such is the condition of the pronephros or head- 

 kidney at the time of hatching. 



When the young fish emerges, the folds of the loop in front 



