aUBKINGDOM METAZOA. 



45 



curs in many parasitic forms and in certain sponges, Flat- 

 worms, Mollusks, and Crustacea, seems to have been second- 

 arily acquired. It is probable that the ancestral Metazoa 

 were unisexual, possessing reproductive elements of only one 

 kind, a supposition borne out by the frequent association of 

 hermaphroditism with a parasitic or sessile mode of life, such 

 conditions being what may be termed abnormal, and usually 

 accompanied by marked structural characters which are to 

 be regarded as secondary modifications. On the other hand, 

 it is noticeable that the lowest free Metazoa (such as the free- 

 swimming Cnidaria) are unisexual. 



An ovum is a single cell, and in its typical form consists 

 of a mass of protoplasm containing a nucleus, and may or 

 may not be surrounded by a membrane. 

 Seldom, however, does such a simple ovum 

 occur; usually more or less yolk, consisting 

 of fatty and albuminous globules, is distrib- 

 uted throughout the protoplasm, and fre- 

 quently the amount of yolk far overbalances 

 the amount of protoplasm. Other structures, 

 such as albumen and one or more enveloping 

 membranes, may be added, the ova of different 

 species differing greatly in this respect. 

 Among the lower forms the ova are usually 

 extruded freely from the body of the parent, 

 but in many of the higher Metazoa they are 

 enclosed within protective cases (cocoons), as 

 in the Earthworm, or imbedded in jelly-like 

 masses, as in the common Pond-snails. 



In the ovary of a young individual all the Fia.20.— Ovarial 

 germ-cells are alike, and all are potentially Tube op a Bbe- 

 reproductive cells; very frequently, however, 

 many of the primitive germ-cells relinquish 

 their reproductive function and serve as pur- 

 veyors of nutrition to certain of their com- 

 rades which enlarge and become mature ova. 

 This is well seen in insects, in which each ovary 

 (Fig. 20) consists of a number of tubes tapering to a point at 

 one end, while at the other they open into a common duct. 



TLE (after LuB 



BOCK). 



g — germinal re- 

 = ova. [gion. 

 o'=matuie ovum. 

 y = yolk-cells. 

 /= follicle-cells. 



