720 LIFE : OUTLINES OF GENERAL BIOLOGY 



of the egg, but in the meantime it is important to recognise 

 that its general type depends on the quantity and the disposition of 

 the yolk. 



The envelopes or investing sheaths of the ovum are of various 

 kinds: (i) made by the ovum itself and of the nature of a cell-wall, 

 the delicate vitelline membrane, which is always present ; (2) formed 

 by adjacent cells, sometimes sister-cells of the ovum, as in many 

 insects, and sometimes arising from the connective tissue of the 

 ovary, the result being a follicular envelope, as in Tunicates, or a 

 firm multicellular vitelline capsule, as in birds; and (3) the product 

 of special glands or groups of glandular cells in the walls of the 

 oviduct, the result being some kind of shell. In the middle of the 

 oviduct of the skate there is a large "o'viductal gland" which secretes 

 through many pores a liquid gluey horn or keratin. This coalesces 

 into a firm shell, "the mermaid's purse", filled with white of egg, 

 and the large ovum floats in the middle. In dogfishes the four corners 

 of the purse form very long tendrils, remaining short in skate's 

 eggs, which twine automatically round and round seaweeds or 

 zoophytes. Thus the egg-case is saved from being smothered in the 

 mud, and its passive movements, as it sways about, may facilitate 

 the aeration of the embryo. When the young fish is well formed and 

 is coming to an end of the nutritive resources in its yolk-sac, a 

 chemical change occurs in the albumen and a sht is formed at one 

 end of the purse, out of which the creature forces its way. Firm 

 sheaths, like these mermaid's purses and the calcareous shells of 

 many reptiles and all birds, must be formed after fertilisation, for 

 no sperms could get through the shell. Yet this must not be regarded 

 as too obvious, since many of the eggs of insects are surrounded 

 before fertilisation by firm chitinous shells, through which no sperm 

 could penetrate. In these cases we find on the chitinous shell a minute 

 aperture or micropyle through which a spermatozoon may enter. 

 Often there are several micropyles, and as the spermatozoa some- 

 times move round and round the shell in varying orbits, one of them 

 is likely to find an entrance. When the fertilisation is in this way 

 effected, a change in the egg-cell may block the openings ; but cases 

 are recorded where several sperms effect entrance into the insect's 

 egg. Egg-shells in the strict sense are, of course, to be distinguished 

 from egg-capsules or cocoons, in which several eggs are wrapped 

 up together. Thus the cocoon of an earthworm contains numerous 

 ova, though it is but rarely that more than one of these succeeds in 

 developing into a miniature worm. Good examples of capsules are 

 the small vases made by the dog- whelk, and here again there is a 

 struggle for existence in the cradle. 



SPERMATOZOON OR MALE CELL.— In typical cases the 

 spermatozoon is very minute and very active. In its minute size, 



