516 SPERM-CELLS. 



rangia, are the result. It is this passage from one thread to the other 

 which betrays the first indications of sex. 



In the second, that is, by filaments, two cells are again necessary, 

 which, differing in construction and also in function, are des- 

 tionsbyfiia- ignated the sperm-cell and germ-cell respectively. Of this 

 ments. ^^ Q there are two modifications : 1st. Reproduction by mov- 



ing filaments, as presented in the higher algse and ferns ; 2d. By elon- 

 gating filaments, as in flowering plants. The moving filaments, which 

 were discovered in the case of animals soon after the introduction of the 

 microscope, were regarded as animalcules, and passed under the designa- 

 tion of spermatozoa. The germ which arises in the first of these modi- 

 fications is, in the lower tribes, unprovided with any nutritive supply ; 

 in the higher, a stock of food is prepared for it by the parent. In. the 

 second, the sperm-cell, or, as it is frequently termed, pollen grain, does 

 not produce a moving filament, but elongates itself into a delicate tube 

 until it reaches the germ-cell. A stock of nutritious matter is placed 

 around the resulting embryo, and this is the ordinary construction of 

 seeds. 



Eestricting our description to the case which more immediately inter- 

 ests us, we shall first consider the mode of origin and nature of the 

 sperm-cell and its filaments in animals, and then of the germ-cell and its 

 process of development when fertilized. 



1st. Of the Sperm-cell. The testes are the organs in which the 

 sperm-cells and filaments arise in man. They are of an ovoid fbrm ; 

 each is covered with a white envelope, the tunica albuginea. A serous 

 membrane, folded as a shut sac, overlies this tunic. From the inner 

 surface a number of delicate projections arise, which divide the organ into 

 several compartments. In these compartments are lodged lobules aris- 

 ing from the tubuli seminiferi and their supplying blood-vessels. There 

 Production of are a ^ out 450 lobules in each testis ; their shape is conical, 

 sperm-ceils by the diameter of the tubes of which they are composed about 

 the 2-^0- of an inch. The total length of this tubular struc- 

 ture is about three quarters of a mile. Before the tubuli of each lobule 

 reach the rete testis, they cease to be convoluted, and bundles of them, 

 uniting into larger vessels, are designated tubuli recti. In the rete tes- 

 tis there are from half a dozen to a dozen of these tubes, which various- 

 ly anastomose with one another and divide. They empty into the vasa 

 efferentia, which, from being straight, become convoluted, a series of 

 cones arising, which together form the globus major of the epididymis. 

 This is a convoluted canal, of about twenty feet in length, which, de- 

 scending, receives beyond its globus minor the vasculum aberrans. It 

 then empties into the vas deferens. 



Fig. 246, human testis : , testis ; , lobes ; <?, tubuli recti ; d, rete 



