LECTUEE XIV 

 REPRODUCTION BY GERM-CELLS. 



Historical — Differentiation of germ-cells into male and female— Pandorina — 

 Volvox — Sperm-cells and ova in Algix; — Zoosperm form of the male germ-cells — 

 Zoosperms of the Barnacles — Adaptation of the sperm-cells to the conditions of fertili- 

 zation — Daphnids— Spermatozoa in different animal groups — Their minute structure- 

 Form and structure of the egg-cell — Adaptation of tlie ovum to given conditions — 

 Dimorphic ova in the same species — Nutritive cells associated with egg-cells — Comi^lex 

 structure of the bird's egg. 



If we now turn to the reproduction of the Metazoa and Meta- 

 ph3^ta by means of germ-cells we find that a great number of lowly 

 plants produce germ-cells which require nothing more for the develop- 

 ment of a new plant beyond certain favourable external conditions, 

 above all, moisture and warmth. Such, for instance, are the ' spores ' 

 of the ferns, which are formed on the under surface of the fronds 

 in little clusters of a brown or yellow colour, easily visible to the 

 naked eye. These spores are individually very small, so that thousands 

 go to form one spore-cluster or sporangium, and millions of spores are 

 given off annually by a single fern. Each spore is a germ-cell 

 enclosed in a protective capsule, and may, if carried by the wind 

 to a spot favourable to germination, become a young plant, the 

 so-called prothallium, from which the fern-plant proper subsequently 

 develops. 



This reproduction by spores has been regarded as a form of 

 'asexual reproduction' so-called, and has been classed along with 

 budding and fission under this head. But it has nothing in common 

 with these forms of multiplication except the negative character that 

 the act of fertilization, which we shall inquire into later on, does not 

 in this case occur. This mode of classification has no longer any 

 more justification than the division of the animal kingdom into back- 

 boned and backboneless animals, in which the negative character of 

 the absence of vertebrae has led to the slumping of quite heterogeneous 

 forms in one group. I do not mean to dispute that both these 

 classifications were fully justified in their own time; indeed they 

 expressed a step of progress. Nowadays, however, the division 

 ' Invertebrata ' or ' backboneless animals ' as a scientific conception 

 has been abandoned, and the same should be done with the category 



