THE EXTERNAL ORGANS 201 



is accompanied by a shortening of the gubernacular cord, which thus appears 

 to draw the organ downwards into the scrotum ; the testicle, following the line 

 originally taken by the gubernacular cord, passes down along the posterior wall 

 of the processus vaginalis, which it therefore invaginates from behind. 



In many animals the testicles remain throughout life in the abdominal cavity. In others 

 they only descend into the scrotum during the period of ' heat.' Cases of cryptorchismus, in 

 which one or both testicles have failed to reach the scrotum, and have remained either in the 

 inguinal canal or within the abdominal cavity, are not unfrequent in the human subject. In 

 rare cases the ovaries may also pass through the abdominal ring by a passage corresponding 

 to the processus vaginalis called the canal of Nuck, and may even be found in the labia majora, 

 where they resemble in position the testicles within the scrotum. 



Fate of the cloaca entodermica : Formation of external genital 

 organs, perineum, and anus (figs. 253, 254, 255). The cloaca entodermica, 

 as we have already seen, is a large chamber connected at its oral end with the 

 allantois and hind-gut, and closed ventrally by the cloacal membrane (fig. 253, A), 

 which extends from the umbilicus to the root of the tail. The cloacal chamber, 

 surrounded by an investment of mesoderm, occupies the whole depth and width 

 of the hinder part of the body- wall. By the increase in the amount of the investing 

 mesoderm, dorsal to and on each side of the cloaca, the body-wall is caused to 

 project between the hind limb buds as an elliptical swelling known as the cloacal 

 tubercle. For some distance behind the umbilicus the mesoderm reaches the mid- 

 ventral line, and an even salient surface is produced, which ends below in an angular 

 projection. This afterwards expands into the genital papilla. Behind the angle the 

 lateral sheets of mesoderm are separated by the cloacal membrane, and produce 

 surface folds, which bound a shallow cleft. This extends towards the root of the tail, 

 but is separated from it by a small projection, also caused by a thickening of the 

 underlying mesoderm (anal tubercle), and by a slight recess behind it which marks the 

 posterior limit of the cloacal elevation. Meanwhile important changes are taking 

 place in the form of the cloacal chamber. Owing to the increase of the mesoderm in 

 the tongue-like projection between the allantois and hind- gut (fig. 253, A) and also 

 at the sides of the cloaca, the openings of the allantois and hind-gut are apparently 

 shifted backwards in other words, a frontal septum takes shape which separates a 

 dorsal or rectal from a ventral or urogenital tube, and reduces the cloaca to a 

 narrow passage between the two (fig. 253, B, C). Whether the process thus sketched 

 involves an actual division of the chamber by lateral folds, or is merely the expres- 

 sion of differential growth such that the ventral part of the chamber, with the 

 Wolfnan ducts attached thereto, expands forwards, while the opening of the gut 

 is shifted backwards to the caudal part of the dorsal wall, is not yet decided. While 

 this urorectal septum is forming, the lumen of the ventral part of the chamber is 

 narrowed to a sagittal cleft, and is encroached on by an epithelial mass which 

 forms a sagittal plate named the cloacal plate. 



The cloacal plate (Kloakalplatte, Keibel ; bouchon cloacal, Tourneux ; Uralplatte, Fleisch- 

 mann) may be looked on as a thickening of the cloacal membrane in the future urogenital 

 part of the cloacal fossa. According to Disse (1905) the epithelial cells are entodermic in 

 origin, the plate being formed by the apposition of the walls of the ventral part of the cloacal 

 chamber. The same view was advanced earlier by Fleischmann (1902-1904). Tourneux, 

 who was the first to describe the plate (1889), looked upon it as an ectodermic thickening, and 

 as representing, when afterwards fissured, the rudiment of an ectodermic cloaca. Otis (1906) 

 inclines to the same opinion. 



The cloacal plate extends as a keel-like thickening into the substance of the 

 cloacal tubercle, and reaches caudally nearly, but not quite, to the anal tubercle, 

 Here the cloacal membrane remains as a thinner lamella, which closes in the dorsal 



