238 



ZOOLOGY 



SECT. 



being little more than a transverse bridge of nerve-matter over 

 the anterior end of the fourth ventricle. The medulla (med.) is of 

 relatively large size. 



Urinogenital Organs. The kidneys are short, being confined 

 to the posterior portion of the body-cavity, and are firmly attached 

 to the ovaries or testes. Each has a thick-walled ureter which 

 joins its fellow, the passages, however, remaining distinct to 

 near the opening into the urinogenital division of the cloaca, 

 when the right opens into the left. 



There are two elongated ovaries (Fig. 861, ov.) which remain 

 distinct throughout. The oviducts (I. ovd. and r. ovd.) are a pair 



.yTt.pl 



FIG. 862. Ceratodus Forsteri. Stages in the development. A, lens-shaped blastula ; B, 

 stage with semicircular blastopore (bl. p.) ; C, later stage in which the blastopore (bl. p.) has 

 taken the form of a ring-like groove enclosing the yolk -plug (yk. pi.) ; D, stage in which the 

 blastopore (blp. sut.) has assumed the character of a longitudinal suture around which is the 

 rudiment of the medullary folds (med.); E, stage in which the medullary folds (med.) have 

 become well developed and enclose the blastopore reduced to a zig-zag slit ; F, later stage 

 with well-formed head with two visceral arches (vise.) and rudiments of eye (eye) and ear 

 (aud.) ; pron. mesonephros. (After Semon.) 



of thick-walled, greatly convoluted tubes which extend along the 

 whole length of the body-cavity, into which they open in front 

 {cod. ap.) ; posteriorly they coalesce immediately before opening 

 into the cloaca. The testes are long, compressed bodies which 

 remain distinct from one another throughout their length. The 

 Mlillerian ducts in the male are remarkably well-developed. 

 There is no vas deferens, and the sperms appear to reach the 

 exterior through the abdominal pores. 



In the early stages of its development (Fig. 862) Ceratodus 

 exhibits resemblances, on the one hand, to Petromyzon (p. 126), and 



