942 



THE SEMINAL FLUID. 



cow Tsenia ccenunis is found in the intestine of the dog, its cysticercus in the brain 

 of the sheep (coenurus cerebralis, the cause of staggers). Taenia echmococcus pos- 

 sesses but two or three segments, a few millimeters long, which are present in 

 innumerable quantity in the intestine of the dog. Its encysted form (acephalo- 

 cyst, with daughter-cysts) often attains the size of a child's head in man. It occurs 

 in the liver, but also, though less frequently, in all other tissues. It is often 

 dangerous to life and it is found also in slaughter- animals. Bothriocephalus 

 latus is found in the intestine of man, its bladder-worm in the meat of the pike. 



Among lower animals the medusae also exhibit alternation of generation: 

 among insects gall-gnats (cecidomyia, with endogenous larval multiplication) 

 and plant-lice. The latter develop in the spring from impregnated, hibernated 

 ova as asexual organisms. These produce successively in numerous generations 

 unfertilized, living, likewise asexual offspring. In the late autumn the last of the 

 young males and females are thus produced, and the latter, impregnated, deposit 

 the fertilized ova. 



Parthenogenesis or virgin reproduction is characterized by the circumstance 



that, in addition to sexual pro- 

 creation, generation without sexual 

 connection may also take place at 

 the same time. The asexually pro- 

 duced brood is always of but one sex. 

 The beehive serves as an example. 

 It contains the queen-bee (sexually 

 mature procreative female) the 

 workers (imperfect females) and the 

 drones (males) . In swarming (nuptial 

 flight) the queen is impregnated 

 by a drone. The semen stored for 

 three or four years of the repro- 

 ductive life in the receptaculum 

 seminis can apparently be added 

 by the queen to the ova to be 

 deposited for purposes of fertilization 

 or be withheld from them. It is 

 also possible that the matter of 

 impregnation depends upon me- 

 chanical conditions related to the 

 size of the comb in which the ova 

 are lodged. Impregnated ova give 

 rise to females only, unimpregnated 

 ova to males only. If the queen is 

 incapacitated for flying and if she cannot be impregnated, she deposits ova that 

 produce drones only. Generous feeding of the larvae of the impregnated ovum, 

 perhaps also the size of its comb (queen-bee cradle), favors the development 

 of a perfect female (queen-bee), while if the nourishment be insufficient, sexually 

 deficient working females result. This description has recently been challenged, 

 it being maintained that the normally impregnated queen always deposits only 

 impregnated ova, whose sexual development depends upon nutritive influences 

 on the part of the workers. 



In many of the higher animals the ova may pass through the first stages of 

 development without impregnation, for example the hen, swine, rabbits, salpians, 

 to the stage of division. Unfecundated ova of starfish even develop to the larval 

 form. 



Sexual reproduction without intermediate forms occurs in mammals, birds, 

 reptiles and most fish. 



THE SEMINAL FLUID. 



The ejaculated seminal fluid is intermixed with the secretion of the 

 alveolar glands of the tubules of the epididymis, the racemose glands 

 of the vas deferens, the glands of Cowper and the prostate gland, and 

 with the fluid of the seminal vesicles. It has a neutral or alkaline 

 reaction and it contains 82 per cent, of water, serum-albumin, alkali- 

 albuminate (propeptone from the accessory glands), nuclein (nucleinic 



FIG. 353. Seminal Crystals. 



