260 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



l:)ody, whereas Stevens stated that there is but one maturation division 

 without reduction. In the second and third cleavage stages (Fig. 182) 

 Toth found no nuclei in the egg center. In the four-nucleate stage the 

 egg follicle still retains its connection with the nutritive chamber by means 

 of plasmatic strands as in the acrotrophic type of egg tube generally 

 (Fig. 182), but shortly afterward the embryo is completely cut off from 

 the nutritive, or end, chamber. In the fourth or fifth cleavage one of 

 the dividing nuclei may lie with the axis of its spindle perpendicular to the 

 egg surface, thus giving rise to a nucleus surrounded by plasma lying in 



Fig. 182. — Aphis samhuci. 

 Cleavage, (fol) Egg follicle. 



myct 



Fig. 183. — Myzus galeop- 

 sidis. Blastoderm. Eighth 

 cleavage. Chromatin elimina- 

 tion in the mycetoblasts. 



or near the center of the egg. Up to this time cell division has been 

 synchronous. 



In the seventh cleavage stage near the posterior end of the egg are 

 several cells, distinguished by their larger size and more abundant chro- 

 matin, that have not divided (Fig. 183). Here one finds three kinds of 

 cells: blastoderm and yolk cells distinguishable from each other only by 

 their position and the larger cells noted above which represent the future 

 mycetocytes (Fig. 183, myct). The cell walls begin to appear in this 

 stage. The future mycetocytes next unite into a syncitial mass in the 

 posteromedian part of the egg. By this time the blastoderm is completed, 

 and the cell walls are distinct (Fig. 184). Two types of blastulae may 

 occur: one completely closed and the other open at the posterior end 

 (Fig. 185). These forms may even occur in the same species. As devel- 

 opment in the open type of blastoderm proceeds, the mycetom (syncitial 

 mass of mycetoblasts) in contact with the egg follicle pushes up into the 

 blastocoele. The mycetom anlage in its further growth crowds against 



