. Chordate morphology. Morphology (Animals); Chordata. RANA B BOMBINATOR. vas deferens ampulla ALYTES Figure 10-1 5. Connections between the testis and the nephric duct in amphibians. (After von den Broek) EMBRYOLOGiCAL DEVELOPMENT In the frog there are three pronephric funnels; the first is at the level of the sec- ond myotome and the third visceral arch (Figure 10-16). These funnels develop from the somatic layer of the inter- mediate plate in the usual fashion. The nephric duct is con- tinued posteriorly by segmental crest contributions until it reaches and opens into the cloaca. The anteri
. Chordate morphology. Morphology (Animals); Chordata. RANA B BOMBINATOR. vas deferens ampulla ALYTES Figure 10-1 5. Connections between the testis and the nephric duct in amphibians. (After von den Broek) EMBRYOLOGiCAL DEVELOPMENT In the frog there are three pronephric funnels; the first is at the level of the sec- ond myotome and the third visceral arch (Figure 10-16). These funnels develop from the somatic layer of the inter- mediate plate in the usual fashion. The nephric duct is con- tinued posteriorly by segmental crest contributions until it reaches and opens into the cloaca. The anterior nephrocoels fuse and open widely into the coelom. In this process the most anterior funnel is moved back into the next segment. These funnels are associated with a large medial glomer- ulus which can be observed in the lO-mm frog. The con- voluted tubules of the pronephros lie in the anterior cardinal (venous) sinus. At the 10-mm stage there are no meso- nephric elements, but the blastemas of these can be observed dorsomedial to the nephric duct. The nephric duct has already extended posteriorly to the cloaca. As develop- ment progresses, the pronephric funnels may fuse into a single unit. The "mesonephros" arises from about the 6th myotomic segment back to the 11 th; it is doubtful that any metaneph- ric materials are involved. The tubes of segments 9 and 10 are very advanced as compared with those in front or behind. The number of units increases anteriorly and posteriorly up to about a dozen, but these have no segmental correlation. The most anterior tubule or two loses its renal corpuscles (glomerulus and Bowman's capsule), becomes connected with the testis, and carries only sperm (Figure 10-13). The next three or four primary tubules lose their corpuscles and conduct sperm, but their secondary tubules are excretory. More posteriorly the primary tubules degenerate to collect- ing ducts serving many nephrons (excretory tubules). More posteriorly these collecting
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