DEVELOPMENT OF VERTEBRATES. 399 



Supra-renal Bodies. — These are structures of unknown 

 function, which occur in most Vertebrates near the repro- 

 ductive organs and kidneys. They seem to have a double 

 nature and origin : {a) a nervous part is derived from the 

 sympathetic nervous system, and \V) a non-nervous part is 

 derived from the anterior nephridia (pronephros or even 

 part of the mesonephros), and from a degenerate anterior 

 portion of the reproductive organs. But it does not seem to 

 be certain that what are called supra-renal bodies are homo- 

 logous throughout the series. 



Reproductive System. — The ovaries and testes are developed 

 from a ridge of the epithelium which lines the abdominal 

 cavity. 



The male elements or spermatozoa usually pass from 

 the testes into efferent vessels which open on each side into 

 a vas deferens or Wolffian duct. 



The ova are shed from the ovaries into the body-cavity, 

 but are usually more or less directly received by the Miil- 

 lerian ducts or oviducts. 



For some of the important exceptions, see Cyclostomata 

 and Teleosteans. 



Development. — A ripe ovum in the ovary is usually sur- 

 rounded by a tunic of less successful cells. Tunic and ovum 

 together form a " Graafian follicle," which bursts when the 

 ovum is liberated. 



Round the ovum are various membranes, notably a zona 

 radiata traversed by fine pores, which give the membrane 

 a striated appearance, and outside this a vitelline membrane 

 of a firmer character. But these membranes are often 

 transitory. 



The formation of polar globules has been detected in the 

 maturation of some Vertebrate ova, but not satisfactorily 

 above Amphibians. 



The ova are fertilised outside of the body in Cyclostomata, 

 Ganoids, Teleosteans, Dipnoi, and tailless Amphibians, 

 internally in the other Vertebrates. 



Segnientation is total (holoblastic) in the ova of the 

 lamprey, the sturgeon, the Amphibians, and all Mammals 

 except the Monotremes. It is partial (meroblastic) in 

 the ova of Elasmobranchs, Teleosteans, Reptiles, Birds, and 

 Monotremes. 



