9 8 



THE GERM-CELLS 



Fig. 46. Germ-cells of Volvox. [OvERTON.] 

 A. Ovum (oosphere) containing a large central 



nucleus and a peripheral layer of chromatophores; 



/. pyrenoid. B. Spermatozoid ; c,v. contractile vacu- 



oles; e. "eye-spot" (chromoplastid) ; p. pyrenoid. 



C. Spermatozoid stained to show the nucleus (). 



of the plant-ovum is the fact that it often contains plastids (leuco- 

 plasts or chromatophores) which, by their division, give rise to those of 



the embryonic cells. These 

 sometimes have the form of 

 typical chromatophores con- 

 taining pyrenoids, as in Volvox 

 and many other algae (Fig. 

 46). In the higher forms 

 (archegoniate plants), accord- 

 ing to the researches of 

 Schmitz and Schimper, the 

 egg contains numerous mi- 

 nute colourless " leucoplasts," 

 which afterwards develop into 

 green chromatophores or into 

 the starch-building amylo- 

 plasts. This is a point of 

 great theoretical interest ; for 

 the researches of Schmitz, 

 Schimper, and others have 

 rendered it highly probable 

 that these plastids are persistent morphological bodies that arise only 

 by the division of pre-existing bodies of the same kind, and hence may 

 be traced continuously from one generation to another through the 

 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 

 origin. 



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, as the 

 early observers supposed, but the products of cells pre-existing in the 

 parent body. Kolliker, 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 I865, 1 that the spermatozoon is a com- 

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



1 Arch. Mik. Anat., I., '65. 



