52 COMPARATIVE ANATOMY. 



indeed be regarded as the sole criterion, and it should only have 

 weight when considered together with the greater or less regularity 

 of the lumen. But in examining this question we must bear in 

 mind one thing ; namely, that in all these cases we have to do with 

 spaces which are walled in by connective substances, and that 

 vessels are differentiations of these spaces, and therefore presuppose 

 an indifferent condition. Between the two stages, the differentiated 

 and the undifferentiated, there are all kinds of intermediate steps. 



k) Reproductive Organs. 

 §45. 



The phasnonienon of the multiplication of the individual is 

 primitively closely connected with nutrition. Not only is nutrition 

 the cause of the growth and consequent increase in size of the body, 

 but it gives rise to a condition in which the organism converts the 

 excess of nutrient material brought to it into the means for pro- 

 ducing a new individual. In the lower forms, as in elementary 

 organisms, a process beginning with gemmation, and leading on 

 to fission, is a very common phenomenon. The manner in which 

 multiplication is effected varies with the amount of material which 

 is used by an organism in producing a new organism. 



The phenomena of multiplication by gemmation and spore-for- 

 mation, which are so common in the lower divisions of the Inverte- 

 brata, have some relations to sexual differentiation, which indeed 

 does occur among the Protista. It is derived from a stage in which 

 two similar germ-cells fuse to form a new organism (Conjuga- 

 tion). As the two uniting cells become gradually dissimilar they 

 become distinguished into egg- cells and sperm-cells; these are 

 the morphological elements of the sexual reproductive matter 

 throughout the whole Animal Kingdom, notwithstanding their 

 numerous modifications, which are seen most markedly in the 

 seminal cells. While the ovum retains its most essential characters, 

 and can be recognised as such in every division, the seminal cell 

 very early undergoes considerable changes. Like other cells it gets 

 a flagellate process, which may be greatly developed, while the cell- 

 body and its nucleus are so reduced that they ordinarily form a 

 structure of no great size. In this way filamentous structures — 

 spermatozoa — are formed from the seminal cell. Sexual Repro- 

 duction, then, does not exhibit a real contrast to the asexual. 



§ 46. 



We do not exactly know in all cases what are the relations 

 between the place where the reproductive matters are formed and 

 the early rudiments of the body, but from what has been observed 



