236 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



in Anura commonly 3, in Urodeles commonly 2 (in Amphiuma 3 

 according to Field). Probably here as elsewhere the number is 

 really a variable one. As the tubules develop they show active 

 increase in length so that they become much coiled and the same 

 applies to the part of the archinephric duct lying in the pronephric 

 region. 



It is only when they first appear that the nephrocoeles show a 

 segmental arrangement : later on they become merged in the general 

 splanchnocoele. Along the inner wall of the dorsal portion of this 

 cavity, i.e. the portion which represents the fused nephrocoeles, the 

 glomerulus develops as a continuous laterally projecting fold of 

 splanchnic mesoderm. Usually the portion of the body cavity con- 

 taining the glomerulus becomes for a time incompletely shut off 

 from the rest to form a secondary pronephric chamber as in Lung- 

 fishes, the mesoderm covering the lungs undergoing fusion with 

 that covering the bulging surface of the pronephros. The secondary 

 pronephric chamber may in turn be subdivided by the edge of the 

 glomerulus fusing with the mesoderm covering the pronephros. 



Meeoblastic Vertebrates. — As a rule, in the Meroblastic 

 Vertebrates the pronephros never becomes a functional organ, and 

 correlated with this it shows a reduction in its structure. Possibly, 

 as already indicated, this may be due to the presence of the large 

 yolk-sac with highly vascular surface in contact with the external 

 medium, which will facilitate the getting rid of excretory material 

 by diffusion outwards. 



Elasmobranchii. — In Elasmobranchs the ventral ends of certain 

 of the anterior mesoderm segments, usually commencing with 

 segment VII, become dilated to form vesicular cavities (van Wijhe, 

 1889) which are probably to be interpreted as nephrocoeles. The 

 tubule rudiments appear as thickenings of the somatic wall of 

 these nephrocoeles which grow outwards and being in close apposition 

 form at their outer ends, apparently by fusion, a solid continuous 

 pronephric swelling. The tubule rudiments make their appearance 

 in sequence from before backwards. 



Different workers vary in their statements as to the number of 

 rudiments in different forms [Scyllium, 5 — Riiekert, 3 — van Wijhe; 

 Pristiurus, 5 — ■Riiekert, 4 — Rabl, 3 — van Wijhe; Raia clavata, 5 — van 

 Wijhe; R. alba, 8— Rabl ; Torpedo, 7— Riiekert (Fig. 130)] from which 

 we may conclude safely that the number of tubule rudiments is very 

 liable to variation both as between different species and different 

 individuals. This variability may be taken in correlation with the 

 fact, observed by van Wijhe, that in Pristiurus dilated nephrocoeles 

 made their appearance from segment I to segment XIV, gradually 

 diminishing in size towards the end of the series, although tubule 

 rudiments appeared only in 3 segments. Both phenomena indicate 

 that the pronephros in Elasmobranchs as in other groups has under- 

 gone reduction from a once much greater anteroposterior extension. 



In a comparatively late stage the tubule rudiments develop their 



