108 GENERAL EMBRYOLOGY 



smaller among forms in which the sperm are directly or indirectly intro- 

 duced into the reproductive cavities of the female. Some of the Crus- 

 tacea afford interesting illustrations of this. In some Ostracods only a 

 few hundred very active spermatozoa are formed; these are inserted 

 directly into the seminal receptacles of the female. They are most 

 remarkable for their size 2 mm. in length in a few, or more than twice 

 as long as the body of the male. In Daphnia the number of sperm may 

 be only twenty, or even less, six to eight in some species. These too are 

 liberated directly into the brood cavity of the female, which forms only 

 two eggs at a time; these sperm are very adherent, and are said to be 

 somewhat amoeboid. Indeed the spermatozoa of many of the Crustacea 

 are unusually interesting on account of their atypical form and behavior 

 (Fig. 52). Some (Bythotrephes) are quite like ova, large (0.1 mm.), 

 rounded, and quiescent, depending upon a peculiar viscid or adherent 

 quality for their likelihood of attachment to the egg. Others (some 

 Decapods) have a number of stiff radiating processes which seem to 

 function by catching in the hair-like bristles surrounding the openings 

 of the oviducts, where they are placed in amplexus. 



The amount of food yolk contained in the egg is related also to the 

 duration of the embryonic period of development, or to the rate of 

 development, a prolonged embryonic life requiring an abundant supply 

 of food materials, and an unusually rapid rate of development depending 

 upon a supply of easily assimilable nutritive substance. Such a relation 

 is illustrated by the difference between summer and winter eggs, formed 

 by Rotifers, and many Insects and Crustacea; the winter eggs, subject 

 to unfavorable conditions and passing a longer period in development, 

 contain considerably more yolk, and are covered with much tougher 

 and more resistant membranes, than the summer eggs which develop 

 rapidly and under favorable surroundings, indeed often within the brood 

 cavity of the female. Thus in Daphnia the small summer eggs are 

 formed by only three nurse cells (see below) , while the large winter eggs 

 are supplied with food by forty or more nurse cells. When the developing 

 embryo acquires special nutritive relations with the parental tissues, 

 the eggs are of course practically yolkless. 



The provision of egg membranes is associated not only with a reduction 

 in the number of eggs formed, but also with the duration of the em- 

 bryonic period, liability to unfavorable external conditions, prevalence 

 of food-seeking enemies, etc. The membranes which are functional 

 under such conditions are of the secondary and tertiary classes described 

 above. The nature of these varies from the common thin fibrous 

 coverings, to tough and impervious membranes capable of resisting 

 extreme dryness, or the leathery or calcareous " shells " of the Sauropsids. 

 Among the most complex and perfectly adapted membranes or shells, 

 are the egg cases of many Elasmobranchs, and particularly those of 



