NO. 1783. -NORTH AMERICAN LERN^OPODID^— WILSON. 197 



near relatives of Achtheres, the cleavage is discoidal. This means that 

 a single blastoderm cell originally becomes entirely separated from 

 the food yolk, and that this one cell by subsequent division yields the 

 whole blastoderm. On the other hand Korschelt and Heider have 

 suggested that the increase of blastoderm in these cases is really due 

 to the accession of new elements from the interior of the egg. In the 

 present species we may clearly distinguish a combination of these two 

 methods. 



The first cleavage nucleus of the egg divides twice or more, and with 

 some of the cytoplasmic elements found within the yolk migrates to a 

 definite point at the surface (fig. 3). 



In the majority of cases, as already stated, this point is diametri- 

 cally opposite the wall of the egg sack, and, what is more important, 

 it becomes the posterior end of the embryo. 



Schimkevitch (1896) has stated that in Tracheliastes this migration' 

 is toward the wall of the egg sack and toward the places where the 

 eggs touch one another. And he adds in a footnote that the direction 

 of migration is not determined by oxygenotaxis, since in this case it 

 would always be toward the wall of the egg sack. The present species 

 otTers still more lucid proof of this statement since the migration is 

 directly away from the outer egg sack. But it must be kept in mind 

 that this eventually brings the anterior or head end of the embryo 

 nearest the outer wall where there is the best oxygenation. 



At the time of migration small particles appear between the yolk 

 globules, scattered about rather uniformly. These take a blue stain 

 very readily and are therefore different in nature from the yolk. 

 They look exactly like the particles of chromatin in the original 

 cleavage nucleus, and like those which subsequently appear in the 

 blastoderm cells, but there is no means at present of definitely proving 

 their nature. They are not found in the unfertilized egg and they 

 entirely disappear by the time the blastoderm is completed. They 

 are the small black dots seen in figs. 3 to 7. 



Wlien these migrated materials reach the surface they are there 

 transformed into blastoderm cells by an accumulation of the cyto- 

 plasmic material around nuclear centers. At first a single large cell is 

 formed (fig. 4), ovoid in shape, its long diameter one-fourth that of 

 the egg. Around this cell is a considerable mass of surplus material, 

 similar in nature and containing one or more nuclear centers. 



By the differentiation produced in double staining this material 

 may be followed far down into the yolk of the egg from whence it is 

 migrating. The large cell, however, in this as w^ell as in subsequent 

 stages, is entirely separate from the yolk and simply rests upon the 

 surface of the egg. 



The cytoplasmic material goes on collecting around a second 

 nuclear center and shortly wo find two large cells of practically the 



