126 INVERTEBRATA CHAP. 



much more severe curvature than usual, and these curvatures produce 

 a series of strains which distort the resulting larva. 



A cell of the 8-cell stage is incapable of developing into a 

 Pilidium. When the 8-cell stage is broken in two, its two constituent 

 portions, viz. the macromeres and the first quartette of micromeres, 

 each group of four cells can develop into a larva. But the micromeric 

 group form a larva with a very large apical sense-organ and no gut, 

 whilst the macromeric group develops into a larva with an enormous 

 gut and no apical organ. The same result is obtained by cutting the 

 blastula along the equator, in this case the upper half forms a larva 

 with enormous apical organ and vestigial gut, whilst the lower half 

 forms a larva with large gut and no apical organ. Hence we conclude 

 that whereas every one of the first four blastomeres contains all the 

 substances necessary to form a perfect larva, after the occurrence of 

 the third cleavage the substance necessary for the formation of the 

 gut is restricted to the lower cells, whilst that destined to form the 

 apical organ is confined to the upper four cells. 



Yatsu found that when the fertilized egg is cut into fragments 

 abnormal Pilidia are produced, except where only a small fragment 

 from the animal pole has been removed, and hence he concludes that 

 the material destined to form the apical plate is situated not at the 

 animal pole but in a ring a short distance beneath it. 



When we review the results of these experiments we are struck 

 with the demonstration which they afford of the influence of the 

 materials given off from the nuclei on the cytoplasm, and also with 

 the proof that at the moment when sperm and egg nuclei approach 

 one another a definite structure or arrangement of organogenetic 

 materials is impressed on the cytoplasm. The outward and visible 

 sign of this inward process may be the radiations which extend from 

 the sperm nucleus outwards. This conclusion will be supported by 

 evidence to which we shall call attention during our study of various 

 other invertebrate groups. The structure impressed on the cytoplasm 

 reminds us of what \vas found to be the case with the Ctenophore egg, 

 but it is not so definitely specialized as in the Ctenophore egg. In 

 this respect the egg of the Nemertine occupies an intermediate 

 position between the egg of the Hydromedusan and the egg of the 

 Ctenophore. 



AFFINITIES OF NEMERTINEA. 



We now approach the final question as to what light the develop- 

 ment of Cerebratulus throws on the ancestry of the Nemertinea as a 

 whole. This question resolves itself into the problem : What is the 

 ancestral significance of the Pilidium larva ? We have to interpret a 

 larva with a simple sac-like gut, opening by a mouth at its lower pole, 

 whilst its upper pole is occupied by a cup-like sense-organ carrying 

 long stiff cilia, and its locomotion is effected by a lobed band of cilia. 

 Just as in the case of Mliller's larva we are again reminded of a 

 primitive Ctenophore. Mliller's larva does not carry the apical tuft 



