THE EXCRETORY ORGANS. 



101 



their cells disappearing, and they are replaced by openings, that of the oral plate 

 forming the opening between the mouth-cavity and the pharynx, that of the anal 

 plate forming the primitive anal opening. The anal plate, before it breaks down, 

 makes a considerable growth, forming an epithelial mass which plays an im- 

 portant part in the anatomical modeling of the region. The oral plate disap- 

 pears very early;' the anal plate much later. 



As soon as the head of the embryo has grown so much as to project as an 

 independent part, we find that the oral plate lies on the under surface of the 

 head, a little in front of the heart. The pro-amnion, pro. am., arises from 

 the somatopleure enclosing the heart, ht., so that when the oral plate be- 

 comes perforate, the cavity of the entoderm, Ent., will communicate directly 

 with the cavity enclosed by the pro-amnion, or, in other words, with the per- 

 manent amniotic cavity. 



A similar anal plate at the posterior end 

 of the embryo also lies within the amnion 

 (Fig. 46). This figure is taken from a sheep 

 embryo in a very early stage, so that the 

 anal plate appears to lie on the dorsal side. 

 By the curling ventralwards or the bending 

 over of the tail end of the young embryo the 

 anal plate is gradually transferred or rolled 

 over on to the ventral side, where it per- 

 manently remains. 



Fig. 46. — Longitudinal Section of the 

 Posterior End of a Sheep Embryo 

 OF Sixteen Days. 



Atfin, Amnion. a.m, Anal membrane (or 

 plate), /r.^f, Primitive streak. ^«, Ento- 

 derm. Ach, Archenteron, or entodermal 

 cavity of the embryo. All, Anlage of al- 

 lantois. 7nes, Mesoderm. — [Afler R. 

 Bonnet. ) 



The Excretory Organs. 



No less than three distinct excretory 

 organs are known to occur in vertebrates. 



Of these, the first is termed the pronephros, or head kidney, on account of 

 its position toward the head and in the neighborhood of the heart. It is well 

 developed and the only excretory organ in many fishes and in the larval stages of 

 amphibia. In elasmobranchs, which occupy in this respect an exceptional posi- 

 tion, and in amniota it exists in a rudimentary form only, except as to its duct, 

 which plays an important role in the further development. The pronephros con- 

 sists of a few epithelial tubes which take a somewhat twisting course, but may be 

 said to run, in general terms, transversely. Each tube begins with a ciliated 

 funnel-shaped opening (Fig. 47, /) not far from the median line of the embryo, 

 and ends, after a more or less contorted course, in a longitudinal duct, which, 

 after receiving all of the tubules, runs toward the posterior end of the embryo 

 and opens into the extremity of the entodermal or digestive canal. Opposite the 



