EXPERIMENTAL MORPHOGENESIS 



its vegetative pole, and this part of it is separated at each 

 of the first two segmentations by a sort of pseudo-cleavage, 

 leading to stages of three and five separated masses instead 

 of two and four, the supernumerary mass being the so- 

 called " yolk-sac " and possessing no nuclear elements 

 (Fig. 8). Crarnpton removed this yolk-sac at the two- cell 

 stage, and he found that the cleavage of the germs thus 

 operated upon was normal except with regard to the size 

 and histological appearance of one cell, and that the larvae 



Fio. S. THE MOLLUSC DENTALIUM (after E. B. Wilson). 



a. The egg, consisting of three different kinds of protoplasmatic material. 

 ?/. First cleavage-stage. There are two cells and one "pseudo-cell," the yolk-sac, which 

 contains no nucleus. This was removed in Crampton's experiment. 



originating from these germs were complete in every respect 

 except in their mesenchyme, which was wanting. A special 

 part of the protoplasm of the egg had thus been brought 

 into relation with quite a special part of organisation, and 

 that special part of the protoplasm contained no nucleus. 



GENERAL RESULTS OF THE FIRST PERIOD OF 

 " ENTWICKELUNGSMECHANIK " 



This experiment of Crampton's, afterwards confirmed by 

 Wilson himself, may be said to have closed the first period 



