vi NEMERTINEA 125 



nucleus is situated develops into a larva. But if the same experiment 

 be performed after the nuclear membrane has faded, all the fragments 

 will develop into larvae. It is, therefore, obvious, that when the 

 nuclear membrane fades, some substance must pass into the cytoplasm 

 which confers on any fragment of it the power to develop into a larva 

 if a spermatozoon be added to it. 



If the same experiment be performed after normal fertilization 

 has occurred, only the fragment containing the first invading 

 spermatozoon will develop. All attempts to fertilize the other 

 fragments by adding fresh spermatozoa failed. 



In the majority of cases the developing fragment is the one 

 containing the zygote nucleus ; but in some cases, when the frag- 

 mentation of the egg had occurred before the spermatozoon had reached 

 the nucleus, it is the fragment containing the spermatozoon and 

 not that containing the nucleus which develops, while the fragment 

 containing the latter can be seen to form the polar bodies, but it goes 

 no farther in development. 



Therefore, just as some substance must exude from the egg nucleus 

 which confers on all the cytoplasm the power to form a larva, so we 

 are bound to conclude that some material is given off from the sperm 

 head which inhibits development in the cytoplasm, except when under 

 the influence of the first nucleus. 



When the egg was cut into fragments, however, and a piece was 

 induced to develop, it gave rise to a perfect Pilidium larva of 

 correspondingly reduced size. The segmentation occurred as in the 

 normal larva, though the blastomeres were correspondingly smaller. 

 But when the first two blastomeres of a normal egg were separated 

 from one another, each divided as if it still formed part of the whole 

 egg, it formed two macromeres and two micromeres. The separation 

 was effected by exposing the developing eggs to the influence of 

 artificial sea- water, made up so as to entirely exclude lime ; such 

 water causes the blastomeres to lose their adhesion to one another 

 and to fall apart, owing apparently to an alteration in the physical 

 characteristics of the outermost layer of the cytoplasm. The separated 

 blastomeres are then restored to normal sea- water and allowed to 

 continue their development. 



When one of the first four blastomeres is separated it forms one 

 macromere and one micromere by the first division, and continues to 

 segment as if it formed one-fourth of the egg. Nevertheless in both 

 these cases the half or quarter blastula closes its wound by narrowing 

 and contraction of the edges, and develops into a Pilidium which is 

 perfectly normal but of reduced size. The Pilidium which develops 

 from one of the first four blastomeres, however, has its apical plate 

 displaced forwards, a change which is probably due to the size of the 

 cells, derived from the segmentation of the blastomeres, remaining the 

 same as if they still formed part of a whole egg. Each cell has 

 therefore to form a part of the larva proportionally four times as great 

 as it would normally have done, and so it must be subjected to 



