454 INVERTEBKATA CHAP. 



of ultra-violet rays. These rays kill all the cells they reach, but it 

 often occurred that one cell of a 2 - cell embryo was exposed to 

 the light whilst the other was sheltered by the tin-foil. In this 

 way it could be shown that when P is killed AB develops into a 

 closed vesicle of similar cells, and that when AB is killed P gives rise 

 to a typical ventral cell family just as it does in the uninjured egg. 



Another most interesting result was obtained by centrifuging the 

 unfertilized eggs. These, under the stress of strong centrifugal force, 

 throw off parts of their substance, so that their volume is often 

 diminished by one-half, and yet when subsequently fertilized they 

 give rise to perfectly normal embryos of correspondingly reduced 

 size. 



The conclusion is therefore obvious, that the cytoplasm of the un- 

 fertilized egg is homogeneous, and that its differentiation into definite 

 regions only takes place at fertilization, and thus, in a different way, 

 Boveri arrives at exactly the same conclusion as that which Zur 

 Strassen had already reached. 



The points sought to be elucidated by Zur Strassen's and Boveri's 

 researches are thus seen to be general questions bearing upon the 

 nature and mechanism of development in general. 



The embryonic development of other Nematoda, so far as is 

 known, agrees closely with that of Ascaris and ends in the develop- 

 ment of a similar Rhabditis larva. The larval development varies 

 enormously according to the condition under which the larval life is 

 passed and the animal or plant in which final ripening takes place ; 

 but the study of these life-histories lies outside the scope of this book. 



AFFINITIES OF NEMATODA 



Before concluding, a few words on the question of the affinities of 

 the Nematoda may be in place. On this question the development 

 throws practically no light at all. Of course, in a general way, it 

 may be said that a hollow blastula is formed and that a gastrula 

 arises by invagination ; but the blastomeres are specialized at an 

 unprecedentedly early stage, and yet their specialization is very 

 different from that observed in eggs with spiral cleavage, which are 

 the only other cases where anything like such early specialization is 

 known to occur. Moreover, our knowledge of the complete develop- 

 ment is so faulty that we cannot as yet make profitable comparisons. 

 Zur Strasseu asserted the existence of a single niesodermic rudiment 

 on each side which, at a stretch, might be compared to a niesodermic 

 band ; but his pupil, Mliller, asserts that the mesoderm arises from 

 three distinct sources, viz. from MST, from D, and from the most 

 anterior daughters of C. 



A thorough and exhaustive revision of the normal development 

 is the first pre-requisite for any profitable theorizing on the subject. 



On grounds of comparative anatomy a relationship with the 

 Chaetognatha has been suggested. Both groups have a strong 



