132 PRINCIPLES OF EMBRYOLOGY 



although somewhat nearer the latter. The main studies in this group 

 have been on an ant, Catnponotus (Reith 193 1), and the bee, Apis (Schnetter 

 1934a, b). In the former, the cortex is at fertilisation more or less uniform 

 in thickness over the whole surface of the egg, although perhaps slightly 

 thicker on the ventral side. As early as the stage of nuclear cleavage, 

 however, it becomes differentiated into five zones, by the flow of internal 

 cytoplasm to particular parts of the surface. This earliest stage of differ- 

 entiation is suppressed if the posterior end of the egg is destroyed by 

 cauterisation, which perhaps indicates that it is controlled by something 

 analogous to a formation centre. The most important of the zones is that 

 from which the germ-band arises, which may be compared to the 

 differentiation centre. Cauterisations in the second half of the cleavage 

 period allow the differentiation of the zones to proceed, but the germ- 

 band zone may be shifted somewhat from its normal position. After the 

 cortical zones have once been fully developed, no further regulation seems 

 to be possible; both the formation centre and the differentiation centre 

 have finished their work before the blastoderm is developed. (It is worth 

 remarking that two of the cortical zones in the ant are concerned with the 

 uptake of symbiotic bacteria, which seem to be always present in the egg, 

 and which fmally arrive in the midgut; their function is obscure.) 



In another hymenopteran, the bee Apis, development is even more 

 precocious, since there are clearly marked cytoplasmic regions in the egg 

 before fertilisation. The cortex is thicker on the ventral side, and in the 

 anterior of this side there is a special collection of cytoplasm in which 

 the maturation of the egg nucleus occurs. Slightly posterior to this is the 

 region of the greatest diameter of the egg, and in this neighbourhood 

 the cortex is thicker and there is more internal cytoplasm mingled with 

 the yolk. A column richer in cytoplasm and poorer in yolk extends down 

 the whole centre of the egg. This structure of the egg clearly affects the 

 migration of the cleavage nuclei, which move into an elongated oval. 

 The shape of this is not quite symmetrical, since the nuclei reach the 

 surface first in the anterior ventral region, which can be considered as the 

 analogue of the differentiation centre. As in Platycnemis, this seems to be 

 the focus of a contraction of the internal material of the egg, but in this 

 case the contraction is concerned not so much with the formation of a 

 simple thickening of the blastoderm, but rather with the infolding of the 

 inner layer, which occurs at a slightly later stage. There is in the bee as 

 yet no evidence for the operation of a formation centre preceding the 

 differentiation centre, but if small portions of the anterior part of the egg 

 are removed by cauterisation, the differentiation centre shifts slightly 

 posteriorly and complete but dwarf embryos are formed. By the early 



