CLEAVAGE AND DIFFERENTIATION I27 



for a long time in the interior of the egg, while the peripheral 

 cytoplasm or blastema remains undivided. It is only later that the 

 nuclei, now very numerous, migrate to the surface of the egg, and 

 the cytoplasm becomes partitioned off into blastomeres, forming the 

 blastoderm (see also p. 88). 



Experiments on the regulatory capacity of the insect egg have 

 given different results in the various groups. In the house-fly 

 Musca domestica, the nuclei have already begun to divide when the 

 Qgg is laid, but the cytoplasm is still quite undivided. Nevertheless, 

 all the parts of the cytoplasm are already determined and chemo- 

 differentiated ; damage done to any part of the cytoplasm results in 

 damage to or absence of some definite structure in the developed 

 organism, and no regulation is possible. Here, then, is a clear case 

 of precocious chemo-differentiation of the cytoplasm and mosaic 

 development in which cleavage plays no part at all.^ 



In the ant Campojiotus ligniperda, it has been possible to deter- 

 mine the time at which chemo-differentiation sets in. This is found 

 to coincide with the start of the visible differentiation of the blastema 

 into various regions, such as those of the future embryonic shield, 

 extra-embryonic blastoderm, etc., which takes place before the 

 nucleus has begun to divide at all. Prior to this time, the egg is 

 undetermined and capable of regulation : after this time the cyto- 

 plasm is chemo-differentiated, and development strictly mosaic.'- 



In the dragon-fly Platycnemis pennipes, the time of onset of 

 chemo-differentiation is relatively later, during the blastoderm 

 stage, and the early egg is therefore capable of regulation. It has 

 been possible to obtain a normally proportioned diminutive insect 

 from one (posterior) half of an tgg constricted transversely into 

 two; duplications and triplications of structures after making 

 longitudinal slits in the blastoderm; and two insects from one egg, 

 the blastoderm of which was divided transversely.^ Later on, how- 

 ever, constrictions and injuries result in the development of 

 partial embryos only. In this case, as in that of Camponotus, it has 

 been possible to establish the very interesting fact that the process 

 of chemo-differentiation emanates as a stream from an activating 

 centre, situated near the hinder end of the egg (figs. 60, 84 and 

 122; see also pp. 170, 252). 



1 Reith, 1925; Pauli, 1927. - Reith, 193 1. ^ Seidel, 1926, 1928, 1929. 



