564 WILLIAM PATTEN. 



make their appearance in greater or less numbers, according 

 to the more or less advanced stage of the blastema. 



From the study of the living egg one might readily infer 

 that the nuclei arise as free formations in the protoplasm, 

 since in no case could they be observed to ascend from the 

 depths of the yolk into the " blastema ;^^ and, indeed, Weis- 

 man claimed this to be the case in the Diptera, although later 

 he withdrew his statement. Strictly defined, and in the sense 

 that we use the term, the '' blastema " is a thin, nucleated layer 

 of protoplasm covering the whole outer surface of the yolk, 

 and not divided into distinct cells. It may be remarked here, 

 that it is not impossible, or even improbable, that a " blastema " 

 may occur in some instances without nuclei, although at 

 present this has not been observed to occur. How such a 

 condition could be brought about will appear later. 



The true structure, however, of the '' blastema " can only be 

 learned by the study of carefully prepared sections, and we 

 shall then be convinced that the above definition is correct. 

 Such sections through the long axis of the egg in the early 

 stages of the " blastema " are represented in PI. XXXVIb, figs. 

 1, 2, 3. Here one may observe : (1) spherical masses of yolk 

 more or less modified by contact with each other; (2) yolk 

 masses whose outlines are so modified as to enclose circular 

 cavities; (3) amoeboid cells, each containing a large and 

 sharply stained nucleus ; (4) an irregular network of protoplasm 

 whose amoeboid filaments are usually connected with the cells 

 just described; and (5) a thin layer of protoplasm at the 

 periphery, containing a few nuclei, and continuous with the 

 protoplasmic network in the interior of the egg. Sections of a 

 stage slightly later than the one just described show (fig. 2) 

 that the blastemic layer has increased in thickness, and is 

 supplied with a considerable number of regularly arranged 

 nuclei. Both the protoplasmic network and the cells observed 

 in the previous stage have disappeared from the yolk. In the 

 next stage observed (fig. 3) the protoplasmic layer has become 

 more sharply defined, the nuclei being more numerous and 

 regularly arranged than in the previous stage. The surface of 



