CHAPTER IV 



EARLY DEVELOPMENT 

 THE BLASTODERM 



In the egg before cleavage begins one may distinguish a central 

 nucleus surrounded by protoplasm; a cytoplasmic reticulum with food 

 yolk in its meshes, forming most of the contents of the egg; and a periph- 

 eral cytoplasmic layer, the periplasm. The periplasm is of variable thick- 

 ness and seems to be very much reduced or entirely lacking in certain 

 Orthoptera, Odonata, and Hymenoptera. The cleavage nuclei which 

 migrate toward the surface are surrounded by a layer of cytoplasm and 

 thus may be called "cells." In some cases, at least, it appears that 

 these cells are not independent but are connected by cytoplasmic strands 

 to the reticulum which, in turn, is joined to the periplasm. In some 

 Orthoptera the blastoderm is formed from cleavage nuclei migrating in 

 the periphery from an egg nucleus near the surface of the egg. The first 

 few cleavage divisions are synchronous in Coccidae, Siphonaptera, Dro- 

 sophila, Phormia, Diacrisia, Brachyrhinus, and numerous other forms. 

 Calandra oryzae (Tiegs and Murray, 1938), however, offers an exception 

 in a complete absence of synchronization. There is a lack of uniformity 

 also in the orders of insects as to when and where the cleavage nuclei 

 reach the periphery (Marshall and Dernehl, 1905). In the Holly Tortrix 

 moth (Huie) all cleavage nuclei reach the periphery simultaneously; in 

 Pieris (Eastham, 1927) the blastoderm is first formed at the anterior 

 pole; and in Diacrisia (Johannsen, 1929) it is first formed at both poles. 

 Apparently the exact region of the periplasm at which the cleavage nuclei 

 first reach the surface is conditioned in part by the shape of the egg and 

 in part by the position of the egg nucleus at the time when cleavage first 

 begins rather than by phylogenetic considerations. The reason for the 

 peripheral movement of the nuclei toward the surface is not clear. 

 Miller (1939) regards it as a passive drifting without intrinsic movement 

 due to local changes in and around the cells themselves, the synchronous 

 migration being due merely to a summation of simultaneous, local, nuclear 

 influences. Sehl (1931) suggests that the movement is caused by the 

 attraction that the periplasm exerts upon the nuclei. 



The cleavage nuclei, each surrounded by a layer of cytoplasm, enter 

 the periplasm where they may undergo one or more tangential divisions. 

 The nucleated periplasm before the formation of cell walls has been 



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