144 'i^f^E GEKM-CELl.S 



(Fi,c;s. 71, 88). Here the spermatozoid is twisted into a conical spiral 

 and bears numerous cilia attached along- the upper turns of the spire. 

 The nucleus occupies the lower turns, and attached to them is a large 

 spheroidal cytoplasmic mass, which is cast off when the spermatozoid 

 is set frci^r at the time it enters the archegonium. This, according 

 to Strasbw^er, probably corresj^onds to the basal cytoplasmic mass of 

 Cliara. The upper portion of the spire to which the cilia are attached 

 is composed of cytoplasm alone, as in CJiara. Ciliated spermatozoids, 

 nearly similar in type to those of the higher cryptogams, have recently 

 been discovered in the cycads by Hirase {Giiigko), Ikeno {Cycas), and 

 Webber {Zciinia). They are here hemispherical or pear-shaped bodies 

 of relatively huge size (in Zamia upward of 250 ix in length), with a 

 large nucleus filling most of the cell and a spiral band of cilia making 

 from two to six turns about the smaller end (Figs. 87, 108). 



As will be shown farther on (p. 173), the "anterior" cytoplasmic 

 region of the spermatozoid, to which the cilia are attached, is probably 

 the analogue of the middle-piece of the animal spermatozoon ; and 

 the work of Belajeff, Strasburger, Ikeno, Hirase, Webber, and Shaw 

 gives good ground for the conclusion that it has an essentially simi- 

 lar mode of origin, though we are still unable to say exactly how far 

 the comparison can be carried. The "posterior" region of the sper- 

 matozoid appears to correspond, broadly speaking, to the acrosome. 



C. Origin of the Germ-cells 



Both ova and spermatozoa take their origin from cells known as 

 primordial germ-cells, which become clearly distinguishable from the 

 somatic cells at an early period of development, and are at first exactly 

 alike in the two sexes. What determines their subsequent sexual 

 differentiation is unknown save in a few special cases. From such 

 data as we possess, there is very strong reason to believe that, with 

 a few exceptions, the primordial germ-cells are sexually indifferent, 

 i.e. neither male nor female, and that their transformation into ova 

 or spermatozoa is not due to an inherent predisposition, but is a reac- 

 tion to external stimulus. Most of the observations thus far made 

 indicate that this stimulus is given by the character of the food, and 

 that the determination of sex is therefore in the last analysis a prob- 

 lem of nutrition. Thus Mrs. Treat ('73) found that if caterpillars 

 were starved before entering the chrysalis state they gave rise to a 

 preponderance of male imagoes, w^hile conversely those of the same 

 brood that were highly fed produced an excess of females. Yung ('81) 

 reached the same result in the case of Amphibia, highly fed tadpoles 

 producing a great excess of females (in some cases as high as 92%) 

 and underfed ones an excess of males. The same result, again, is 



