508 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



tail of the spermatozoon ; in some birds the head is 

 not straight, but is twisted into a spiral (Canary). 

 In the Mammalia there is nearly always a rounded 

 head and a long tail. 



The ova of Elasmobranchii and Teleostei, Am- 

 phibia and Sauropsida, are always of some size, the 

 first and the last of these groups having much the 

 largest ova, or eggs most richly provided with food 

 yolk. In Amphioxus and in mammals * the eggs 

 are very much smaller, and, in consequence of the 

 absence of the disturbing element (the food yolk) the 

 early stages of development are very different. In 

 addition to the investing membranes, which are some- 

 times perforated by a micropyle, some eggs are pro- 

 vided with a calcareous coating or shell. 



Calling to mind somewhat the arrangements which 

 obtain in the Arthropoda, the eggs of vertebrates are 

 enclosed in an envelope of cells which owe their 

 origin to the primitive germinal epithelium ; in the 

 Mammalia this cellular investment becomes more than 

 one layer deep, the cavity within enlarges, and the 

 ovum projects into the fluid therein contained ; this is 

 the structure which is ordinarily known as the 

 Oraafian follicle. 



The numbers of ova formed within any one female 

 vary enormously, and, as may be supposed, the 

 largest numbers are found in the lowest forms, or, in 

 fishes ; thus the cod is credited with nine millions, 

 the haddock and plaice with six millions ; the Elasmo- 

 branchs have a very much smaller number, often less 

 than ten ; in the Amphibia there are often a consider- 

 able number, in the Sauropsida the number is smaller, 

 and no mammal has more than about ten matured at 

 the same time, while in many cases the birth of twins 

 even is only an occasional phenomenon. 



These ova become invested in membranes, of 

 * The Monotremata have been lately stated to have large eggs. 



