370 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



teron ribbons is the splanchnic mesoderm (splm), the ectal part in contact 

 with the ectoderm is the somatic mesoderm {som. m). 



The division of the mesoderm into splanchnic and somatic layers 

 takes place over a considerable period of time. The constriction is not 

 evident at 50 hours (Fig. 323). At 60 hours it is very plainly seen (Fig. 

 324), but not until after the eightieth hour is the separation completed. 

 Figure 325 shows the mesenteron ribbons {mge) at this time considerably 

 widened and nearly meeting on the ventral side over the nerve cord. The 

 development of the gonads, fat body, and heart offers no unusual features. 

 The definitive dorsal body wall is completed at the ninety-second hour. 



Blowflies {Calliphora erythrocephala L. and vomitoria L.) 



The eggs of the members of the genus Calliphora are readily obtained 

 by placing a dead bird or mammal out of doors. In a short time the 

 female flies will begin deposition. The eggs will be found in small clumps, 

 especially around the mouth or anus of the cadaver. The total number 

 deposited by a single female is variable and may reach several hundred. 

 The time of development is variable, ranging from 6 to 48 hours depend- 

 ing on temperature. 



Immediately after deposition the egg shows the cleavage nucleus and 

 perhaps its first cleavage stage in the anterior one-third. It has at this 

 time an outer layer of formative periplasm, with scanty protoplasm in 

 the yolk. The cleavage nuclei migrate to the periphery with the centro- 

 some in front (peripheral), but the centrosome is lateral when division 

 takes place. After some irregularity in distribution of the cleavage 

 nuclei they later arrange themselves at a regular distance from the 

 center and simultaneously migrate to the periphery which they reach at 

 nearly the same time. The oosome may be seen in early stages of the 

 egg near the posterior pole where the periplasm has thickened (Fig. 

 327 A, os) during development. Until the cleavage nuclei have nearly 

 reached the periphery they all look alike, but those which pass into the 

 oosome assume a rounded appearance. The oosome breaks up into as 

 many parts as there are cells in contact with it, a part surrounding each 

 cell in the form of a finely granular peripheral crescentic body (Fig. 

 3275). While the other cleavage nuclei form the blastoderm, the germ 

 cells become differentiated and may be readily recognized as such for a 

 long time. They become visible externally, bulging beyond the surface. 

 Contrary to Graber's statement, the same is true for Lucilia, though here 

 they are less protruding. At the stage when the first germ cells free 

 themselves and assume a rounded appearance, there are 15 or more cells 

 present. As development goes on, they increase in number. 



On the approach of the cleavage nuclei at the posterior end, the original 

 peripheral formative protoplasm thickens but becomes thinner again 



