PROBLEMS OF ORGANIZATION 203 



cleavage begins as an infurrowing from the cortical layer. This separates 

 the blastoderm nuclei and incorporates each plus cortical material and 

 material which came from the underlying stratum of cytoplasm into in- 

 dividual cells. The peripheral cells undergo a growth by accretion or by 

 enlargement of their mass thrbugh the incorporation into them of ma- 

 terial which formerly lay in the cytoplasm of the central syncytium. 



Upon completion of the blastoderm the first movements are those of 

 cells along the ventral mid-line and in the regions which later become the 

 cephalic furrow. They are slipping movements and appear to be conse- 

 quences of changes in cell shape and polarity. These movements lead to 

 the inturning of the mid-ventral cells to form a mesodermal tube and to 

 the establishment of the proctodaeal plate beneath the pole cells. These 

 movements are neither immediately preceded nor accompanied by mi- 

 totic activity. Anteriorly the mesodermal tube extends beyond the 

 cephalic furrow and terminates as a T-like structure just posterior to 

 the region which will produce the stomodaeal plate. At its posterior end 

 the mesodermal tube is for a short time confluent with the depression 

 which gradually deepens in the proctodaeal plate to receive the pole cells. 

 The pole cells subsequently migrate through the proctodaeum and sep- 

 arate into two groups which become the gonads. This period of special 

 movements is followed by one of general movement in which the newly 

 established germ band elongates and extends around the posterior pole 

 of the egg onto the dorsal side, while the dorsal and lateral regions of 

 the blastoderm fold and stretch in making way for the elongating germ 

 band. During this period the proctodaeal plate invaginates deeply and 

 mitoses are found among its cells. Simultaneously at the anterior end 

 certain cells at the level of the stomodaeal plate migrate beneath the 

 ectoderm and undergo mitosis to form a group of cells which become the 

 anterior rudiment of the mid-gut (endoderm). The origin of the pos- 

 terior rudiment of mid-gut is somewhat obscure, but probably of a simi- 

 lar nature, as there is no evidence of its formation from the mesodermal 

 tube. With the invagination of the stomodaeal plate the anterior rudi- 

 ment of mid-gut begins to extend by mitosis and cell movement pos- 

 teriorly and ventrally between the mesoderm (which has now become 

 a flattened layer of cells with no remnant of the original lumen) and the 

 yolk. A similar growth and extensions of the posterior rudiment lead 

 eventually to their fusion to complete the mid-gut. 



A period of special cellular movement begins during the invagination 

 of the stomodaeal plate, when neuroblasts begin to be differentiated and 

 move from the ectoderm lateral to the ventral mid-line into the interior 



