ELEMENTARY MORPHOGENESIS 33 



in the relation between their protoplasm and nucleus : in all 

 eggs it is the protoplasm which is comparatively very large, 

 if held together with somatic cells, in the spermatozoon it 

 is the nucleus. A large amount of reserve material, destined 

 for the growth of the future being, is the chief cause of 

 the size of the egg-protoplasm. The egg is quite or almost 

 devoid of the faculty of movement, while on the contrary, 

 movement is the most typical feature of the spermia. Its 

 whole organisation is adapted to movement in the most 

 characteristic manner : indeed, most spermatozoa resemble 

 a swimming infusorium, of the type of Flagellata, a so-called 

 head and a moving tail are their two chief constituents ; 

 the head is formed almost entirely of nuclear substance. 



It seems that in most cases the spermatozoa swim 

 around at random and that their union with the eggs is 

 assured only by their enormous number ; only in a few 

 cases in plants have there been discovered special stimuli of 

 a chemical nature, which attract the spermia to the egg. 



But we cannot enter here more fully into the physiology 

 of fertilisation, and shall only remark that its real significance 

 is by no means clear. 1 



THE FIRST DEVELOPMENT PROCESS OF ECHINUS 



Turning now definitively to the special kind of organism, 

 chosen of our type, the common sea-urchin, we properly 



1 The older theories, attributing to fertilisation (or to "conjugation," i.e. 

 its equivalent in Protozoa), some sort of "renovation" or "rejuvenescence" 

 of the race, have been almost completely given up. (See Calkins, Arch, fur 

 Entwickelungsmechanik, xv. 1902). R. Hertwig recently has advocated the 

 view, that abnormal relations between the amounts of nuclear and of proto- 

 plasmatic material are rectified in some way by those processes. Teleologically, 

 sexual reproduction has been considered as a means of variability (Weismann), 

 but also as a means of preserving the type ! 



3 



