254 ZOOLOGY SECT, xm 



separated except posteriorly, where they are united by a narrow 

 commissure. The contained cavity is divided into two by the 

 prolongation of the choroid plexus already referred to. The nervous 

 wall of the hemisphere (pallium) is very thin and is incomplete 

 dorsally and internally : basally it forms a massive tuberculum 

 olfactorium from which the olfactory nerve-fibres are derived. 

 There is a pair of large olfactory lobes (rh.), each with its cavity, 

 and each prolonged into an olfactory peduncular tract, which ends 

 in front in an olfactory bulb in close apposition with the nasal 

 capsule. 



The pineal body is situated on the summit of a conical mem- 

 branous cap on the roof of the third ventricle. The iufundibulum 

 develops a pair of lobi inferiores. The mesencephalon (mesa.) is 

 bilobed, but the division is not strongly pronounced. The cere- 

 bellum (cbl.) is very small, 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. 911, I. ov., r. ov.,) which 

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

 pair 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 

 efferent ducts from the testes open into certain of the tubules of 

 the mesonephros, and the sperms are thus enabled to pass out 

 through the mesonephric duct. The Miillerian ducts in the male 

 are remarkably well developed. 



In the early stages of its development (Fig. 912) Ceratodus 

 exhibits resemblances, on the one hand, to Petromyzon (p. 135), and 

 on the other to the next class to be studied the Amphibia. The 

 ova become enclosed, while passing down the oviduct, in a gela- 

 tinous envelope which swells up considerably when it comes in 

 contact with the water. At what stage fertilisation takes place 

 is not exactly known. Segmentation is complete and unequal, 

 and results in the formation of a lens-shaped blastula (A) with 

 smaller cells on one of the convex surfaces (the future dorsal), and 

 larger on the other (the future ventral). A blastopore (U. p.} 

 first appears on the ventral surface as a short transverse slit, 

 which grows into a semicircle (B) or a horse-shoe. The free ends 

 of this grow in towards one another and unite to enclose an 



