xv NEMATODA 451 



Dentalium (see p. 325) and still more strongly in the development of 

 the simple Ascidians as worked out by Conklin. If the objection be 

 raised that it is hard to see why if the zygote nucleus has such a 

 profound effect on the cytoplasm its daughter nuclei should not have 

 an equal effect, it must be answered that there is definite evidence in 

 Echinoderm development that they do not have any effect, that, in 

 fact, coincidently with the entrance of the spermatozoon and the union 

 of the two sexual nuclei, a rearrangement of substance takes place 

 which has no parallel in later development. It is suspected that the 

 separation of ectoderm, endoderm, and mesoderm, which forms the 

 first step in development, the so-called "formation of layers" may find 

 its cause in this rearrangement of cytoplasmic material ; and the well- 

 known circumstance that bud development does not follow the lines 

 of embryonic development (vide Chaps. XI., XVII.) may be due to the 

 fact that the nuclei of a bud-rudiment revert to the zygotic condition 

 and rearrange the surrounding cytoplasm but in a different manner. 



Whether Zur Strassen's explanations are justified in detail or not, 

 which is very doubtful, it is difficult to escape the cogency of his 

 reasoning that some such differentiation must exist, even in appar- 

 ently homogeneous cytoplasm, in order to account for the direction of 

 spindles and the movements of cells ; unless we are going to have 

 recourse to an imaginary indwelling spirit, the Entelechy of Driesch, 

 to account for these things. 



Boveri (1910) has also made investigations on the abnormal 

 embryos of Ascaris, but the type of abnormality which he has chosen 

 to investigate is different from that to which Zur Strassen has devoted 

 his attention. Boveri has chosen for his special subject doubly 

 fertilized eggs. These are to be found in small numbers in almost 

 every lot of Ascaris eggs submitted to investigation. They are 

 recognized by the fact that at the first cleavage they divide into four 

 equal cells. One of the male nuclei fuses with the female nucleus, 

 and as usual brings in its own " cytocentre " or centrosome, and the 

 other male nucleus forms an independent cytocentre, so that at the 

 first division four cytocentres are formed and the egg divides into 

 four cells. 



Now Boveri has proved that there are three varieties of these 

 doubly fertilized embryos. In one variety one of the four cells acts 

 as P, and in subsequent cleavages gives rise to a perfectly normal 

 ventral cell family, whilst the other three act as AB's, and each 

 divides as the upper cell in a normal egg would have done (Fig. 

 354, A). The product of all three coalesce to form a much too 

 voluminous cell cap of " primary ectoderm," but there are indications 

 that embryos of this type occasionally give rise to normal larvae. 



In the second variety two of the four cells act as P's, and two 

 as AB's. The result is the production of a "parallel twin" (Fig. 

 354, B). Development goes on for a certain distance, but the two 

 ventral cell families interfere with each other's expansion, an 

 irregular mass of cells results, and then death supervenes. 



