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William S. Marshall and Paul H. Dernehl. 



will now endeavour to trace their progress into the Keimhautblastem 

 and the formation by them of the blastoderm. 



But little seems to be known concerning the entrance of the 

 cleavage nuclei into the Keimhautblastem and the part played in 

 the formation of the blastoderm cells by the cytoplasm which 

 surrounds each of these nuclei. Metcznikow (40) thought that in 

 Äpkis, and earlier the same for Cecidomyia, that the protoplasm of 

 the blastoderm cells was formed »aus dem Dotterplasma«. Bloch- 

 mann (2) found that in the eggs of Musca, when the cleavage nuclei 

 and surrounding cytoplasm reached the Keimhautblastem, the forward 

 end of each fused with it. Carriere and Bürger (11) for Chalico- 

 doma, found that the protoplasm surrounding the cleavage nuclei 

 fuses with the Keimhautblastem as soon as it reaches it, and Koack 

 (42) for Calliphora, describes these nuclei as carrying their surroun- 

 ding cytoplasm with them when they reach the outer layer, but fusing 

 with it. The best figures on this subject, much fuller thau the text, 

 are those by Carriere and Bürger for Anthophora. In Fig. 16 

 we show a resting nucleus lying just inside the Keimhautblastem, 

 the inner boundary of which is seen to be indistinct, passing gradually 

 into the protoplasm surrounding the nucleus. For the first time the 

 nuclei have now taken a definite position within the surroundiüg 

 cytoplasm, being near the forward margin, that which is nearest the 

 egg's surface. The whole thus assumes a comet-like appearance, the 

 nucleus the head, and the cytoplasm the tail of the comet following 

 behind. Keachiug the nearest part of the Keimhautblastem the nucleus 

 with its surrounding cytoplasm enters it, not however at first fusing, 

 but remaining distinct. In many of our preparations there is no 

 division between the cytoplasm of the cleavage nuclei and the Keim- 

 hautblastem; a number of other slides showed that there was un- 

 doubtedly, for a time, no union between the two. In Fig. 27 we have 

 a view of three cleavage nuclei entering the Keimhautblastem, the 

 upper one the least advanced of the three. It is evident that no 

 fusion has as yet taken place, the cytoplasm around the nucleus 

 can, in these preparations, be distinguished from the surrounding 

 Keimhautblastem by its darker shade. Such a Separation was not 

 seen in all our preparations. The ultimate fate of the cytoplasm 

 brought with the cleavage nucleus is a fusion with the Keimhaut- 

 blastem, the two forming with the nucleus the contents of a blasto- 

 derm cell. Graber (18) holds that by Lucilia the blastoderm cells 

 are formed from the cytoplasm the cleavage nuclei bring with them; 



