GERM CELLS AND THEIR FORMATION 107 



male (Fritsch) (Fig. 54). In Diplozoon (Trematoda) two hermaphrodite 

 individuals, at first entirely separate, become permanently fused so that 

 the openings of the reproductive ducts are in apposition. In Syngamus 

 (Xemathelminthes) also, the male lives permanently attached to the 

 female. The climax of this relation is afforded by Trichosomum (Nema- 

 thelminthes) where several (2-5) dwarfed males live within the uterus 

 of the female (Leuckart) . Or it might be said that the climax is to be 

 seen in the cases of self-fertilizing hermaphrodites; these are usually, 

 internal parasites, where the chances of the , meeting of males and 

 females would be practically nil (e.g., many of the Trematodes and 

 Cestodes). 



The retention of the eggs during their development, within a brood 

 cavity is primarily a protective arrangement, but it often leads to the 

 establishment of an organic nutritive relation between the embryos and 

 the wall of the cavity. This is the case in some Tunicates, Elasmo- 

 branchs, etc., and of course it reaches its climax in the intimate and 

 extensive organic relation between embryo and oviducal (uterine) wall 

 among the Eutherian Mammalia. 



The number of eggs produced during each reproductive or spawning 

 period varies enormously, and is related to a variety of conditions of 

 development. In general the number of eggs is larger when there are 

 few or no other means of ensuring the complete development of a 

 number of organisms sufficient to maintain the species numerically. 

 Any structure of the egg or habit of deposition, adapted to ensure 

 development, is likely to be associated with a reduction in the number 

 of eggs formed. The number is largest among forms which discharge 

 the eggs at random or where they are subject to unfavorable external 

 conditions, to liability to the attacks of parasites, or use as food by other 

 organisms. For example, the marine fishes produce very large numbers 

 of pelagic ova, the codfish is said to form 8 to 10 millions in one season ; 

 and in a species of sea-urchin (Echinus) a single female is said to have 

 discharged, in a single season, as many as 20 million ova. Where very 

 special conditions of development are essential, as in the complicated 

 life histories of many internally parasitic worms, the number of eggs is 

 very large and fertilization is ensured by hermaphroditism. 



The number of eggs is smallest where they develop within a brood 

 cavity, or where some degree of parental care is exercised. In a few 

 Mammals and Birds only a single egg is formed at each breeding period, 

 and in these groups the number rarely exceeds eight or ten. Further 

 the number of eggs produced, in general varies inversely with the 

 amount of food substance contained, or with the chances of the young 

 .finding food for themselves by the time they become free living. 



The number of sperm cells formed is always larger than the number 

 of eggs, and often reaches many millions. The number is likely to be 



