208 



EMBRYOLOGY 



it is found to be full of spermatozoa. The latter present 

 the typical appearance of flagellate seminal filaments 

 (Metschnikofb'). 



It has not yet been observed in what manner fertilization 

 takes place. Julin saw that the superficial cells of the male 

 detached themselves, and that in this way the spermatozoa 

 became free. Since males and females swim about free in 

 the water, it is possible that the seminal filaments penetrate 

 into the female, and that consequently fertilization is inter- 

 nal. The eggs destined to produce females develop inside 

 the " plasmodial sacs," those producing males, free in the 

 body cavity of the AmpJiiura. The statements of authors 

 diifer greatly regarding the embryonic development. 



Development of the Male. — According to Julin, there 

 arises as the result of the unequal cleavage an epibolic gas- 



trula (comp. Fig. 98 A 

 and B), the inner layer 

 of which is at first re- 

 presented by only one 

 large cell. Later, cells 

 are separated from this 

 above and below (Fig. 

 98 0). While the large 

 central cell, by sub- 

 sequently dividing 

 many times, becomes 

 the fundament of the 

 testis, the muscle fibres 

 arise from the cells 

 that were previously sepai^ated off from it, and which at 

 first rest upon it in the form, of a cap (Fig. 98 B and E). 

 The larva assumes the type of the adult animal as the result 

 of the appearance of the chai'acteristic division of the sur- 

 face of the body into rings, the loss of cilia on the second 

 ring, and the formation in it of the highly refractive 

 bodies. 



Fig. 98.—^ to E, stages in the development of 

 the male of Bhopnlwra Giai-AU (after Jdlin) ; H, 

 testis. 



According to Metschnikoff, an epiboly does not take place, but there 

 arises a solid heap of cells of rather uniform size, from which the outer 

 layer and the genital fundament are subsequently differentiated. On the 



