134 ^^^ GERM-CELLS 



plastids are persistent morphological bodies that arise only by the 

 division of preexisting bodies of the same kind, and hence may be 

 traced continuously from one generation to another through the 



/ 



K 

 ^ 



n 



A C^ 



Fig. 64. —Germ-cells of Volvox. [OvERTON.] 

 A. Ovum (oosphere) containing a large central nucleus and a peripheral layer of chromato- 

 phores ; /. pyrenoid. B. Spermatozoid ; c.v. contractile vacuoles ; e. " eye-spot " (chromoplastid) ; 

 /. pyrenoid. C. Spermatozoid stained to show the nucleus («). 



germ-cells. In the lower plants (Algae) they may occur in both germ- 

 cells ; in the higher forms they are found in the female alone, and in 

 such cases the plastids of the embryonic body are of purely maternal 



ongm. 



B. The Spermatozoon 



Although spermatozoa were among the first of animal cells ob- 

 served by the microscope, their real nature was not determined for 

 more than two hundred years after their discovery. Our modern 

 knowledge of the subject may be dated from the year 1841, when 

 Kolliker proved that they were not parasitic animalcules, asfKe early 

 SbseTvers supposed, but the products of cells preexisting in the 

 parent body. KolHker, however, did not identify them as cells, but 

 believed them to be of purely nuclear origin. We owe to Schweigger- 

 Seidel and La Valette St. George the proof, simultaneously brought 

 forward by these authors in 1865,^ that the spermatozoon is a com- 

 plete cell, consisting of nucleus and cytoplasm, and hence of the same 

 morphological nature as the ovum. It is of extraordinary minute- 

 ness, being in many cases less than yooVoo ^he bulk of the ovum.2 



1 Arc/i. Mik. Anat., I. '65. 



2 In the sea-urchin, Toxopiteiistes, I estimate its bulk as being between 40 oVoo ^"^ zomws 

 the volume of the ovum. The inequality is in many cases very much greater. 



