VERTEBRATA. 357 



All the viscera except the nervous centres are contained in a body 

 cavity or coelome inclosed between the body wall and their outer surfaces. 

 It is lined by an epithelium and a delicate layer of connective tissue known 

 as peritoneum. The viscera are kept in position and are sometimes freely 

 suspended by dorsal folds of this peritoneum, known by special names 

 according to the organ they support, mesogastrium (stomach), mesentery 

 (intestine), mesorchium (testis), mesovarium (ovary) ; besides several liga- 

 ments, e. g. those of the liver. Properly speaking the coelome is a paired 

 cavity (p. 334), but traces of its paired character, due to the presence of a 

 ventral in addition to a dorsal mesentery, are rarely found, e.g. in Dipnoi. 

 A section of the coelome is inclosed to form the pericardium, and in Mam- 

 malia a thoracic region, including heart and lungs, is separated from an 

 abdominal region containing the remaining viscera, by a fibro-muscular 

 diaphragm. The coelome usually communicates with the lymphatic 

 system. And it may open externally in one of three ways : by posteriorly 

 placed abdominal pores in Pisces, with the exceptions of some Elasmo- 

 branchii and the vast majority of Teleostei, or by peritoneal canals in some 

 Chelonia and the Alligator : through the nephrostomata when present : and 

 through the Miillerian ducts of the female. 



Each ovum in Vertebrata is surrounded during growth in the ovary by 

 one or more layers of abortive ova, the cells of the tunica granulosa, the 

 whole being known as the Graafian follicle. It is impregnated externally 

 to the body in Cyclostomi, Ganoidei, Teleostei and Dipnoi, and in Anura 

 among Amphibia ; internally in all other Vertebrata. There are three 

 ovular membranes known, (i) an external or vitelline persistent in Aves, 

 (2) a middle striated membrane or zona radiata persistent in Teleostei and 

 Mammalia and probably Amphibia, and (3) a delicate internal membrane 

 present in Reptilia and Mammalia (? in Teleostei). But when ripe the 

 ovum may be devoid of membranes as in Elasmobranchii. A single micro- 

 pyle is present in Petromyzon, and many Teleostei, several in Acipenser. An 

 adventitious coat of albumen is secreted round the ovum in Elasmo- 

 branchii, Holocephali, Dipnoi (Protopterus\ Amphibia, Sauropsida and 

 some Mammalia, to which may be added an egg-shell as in Elasmo- 

 branchii, Holocephali, and Sauropsida. The ovum contains much food yolk 

 in Elasmobranchii, Holocephali, and Sauropsida ; also in Teleostei. It is 

 large in the three first named groups, small in the last named, but segmen- 

 tation is partial in them all. The ovum is smaller in other groups, and 

 segmentation is either very unequal or, as in Mammalia, nearly equal. The 

 gastrula is much modified. In Petromyzon, Acipenser and Amphibia there 

 is a distinct invagination ; one less distinct in Elasmobranchii', while a 

 trace only of it is preserved in Sauropsida and Mammalia in the primitive 

 streak which lies at the posterior end of the medullary groove. 



There is an external yolk sac opening into the intestine with which its 



