Fig. 393. Diagram of the arrangement of the urinogenital organs in an adult male Elasmobranch.
m.d. rudiment of Müllerian duct; w.d. Wolffian duct, marked vd in front and serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; t. testis; nt. canal at the base of the testis; VE. vasa efferentia; lc. longitudinal canal of the Wolffian body.
In the male the following parts are present ([fig. 393]):
(1) The Müllerian duct (m.d), consisting of a small rudiment attached to the liver, representing the foremost end of the oviduct of the female.
(2) The mesonephric duct (w.d) which precisely corresponds to the mesonephric duct of the female, but, in addition to serving as the duct of the Wolffian body, also acts as a vas deferens (vd). In the adult male its foremost part has a very tortuous course.
(3) The ureter (d), which has the same fundamental constitution as in the female.
(4) The segmental tubes (s.t). The posterior tubes have the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them to act as transporters of the semen.
Connected with the anterior tubes there are present (1) the vasa efferentia (VE), united on the one hand with (2) the central canal in the base of the testis (nt), and on the other with the longitudinal canal of the Wolffian body (lc). From the latter are seen passing off the successive tubuli of the anterior segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in my diagram.
Apart from the absence of the pronephros the points which deserve notice in the Elasmobranch excretory system are (1) The splitting of the segmental duct into Wolffian (mesonephric) and Müllerian ducts. (2) The connection of the former with the mesonephros, and of the latter with the abdominal opening of the segmental duct which represents the pronephros of other types. (3) The fact that the Müllerian duct serves as oviduct, and the Wolffian duct as vas deferens. (4) The differentiation of a posterior section of the mesonephros into a special gland foreshadowing the metanephros of the Amniota.
Cyclostomata. The development of the excretory system amongst the Cyclostomata has only been studied in Petromyzon (Müller, Fürbringer, and Scott).
The first part of the system developed is the segmental duct. It appears in the embryo of about 14 days (Scott) as a solid cord of cells, differentiated from the somatic mesoblast near the dorsal end of the body cavity. This cord is at first placed immediately below the epiblast, and grows backwards by a continuous process of differentiation of fresh mesoblast cells. It soon acquires a lumen, and joins the cloacal section of the alimentary tract before the close of fœtal life. Before this communication is established, the front end of the duct sends a process towards the body cavity, the blind end of which acquires a ciliated opening into the latter. A series of about four or five successively formed outgrowths from the duct, one behind the other, give rise to as many ciliated funnels opening into the body cavity, and each communicating by a more or less elongated tube with the segmental duct. These funnels, which have a metameric arrangement, constitute the pronephros, the whole of which is situated in the pericardial region of the body cavity.