196 THE EMBRYOLOGY OF THE HONEY BEE 



arranged groups of cells lying laterad to the heart among the 

 fibres of the dorsal diaphragm of Forficula. Although at first 

 sight it may seem somewhat doubtful whether the scattered cells 

 of the dorsal diaphragm in the larvae of the honey bee are to be 

 regarded as the homologues of the segmentally arranged cells 

 composing the "paracardiale Zellstrang" of Forficula and Chali- 

 codoma, nevertheless as will appear later they have precisely 

 the same origin in all three insects, so that their homology can 

 scarcely be open to question. In view of the scattered distribu- 

 tion of these cells in the honey bee it will be more convenient to 

 refer to them simply as the paracardial cells. 



The aorta into which the anterior end of the heart opens, is 

 merely the space around the oesophagus bounded externally by 

 the inner walls of the coelomic sacs of the antennal segment (Fig. 

 42, Ao). It opens cephalad and ventrad into the cavity of the 

 head. 



The ventral diaphragm is composed of a layer of transversely 

 arranged muscle fibres which combine to form a more or less con- 

 tinuous membrane (Fig. 75, VDph) overarching a space just 

 dorsad of the ventral cord, the ventral sinus. The ventral dia- 

 phragm extends from the first to the neighborhood of the twelfth 

 trunk segments, its lateral margins being attached to the mesial 

 margins of the ventral longitudinal muscles and also to the silk 

 glands. In the mid-line its dorsal surface is in contact with the 

 ventral wall of the mid-intestine. 



The blood corpuscles are large rounded cells which at Stage 

 XV are for the most part confined to the lumen of the heart. In 

 size they approach the oenocytes, but their general appearance is 

 quite different. Both nucleus and cytoplasm are pale, the latter 

 being much vacuolated and frequently enclosing deeply stained 

 granules. A group of blood corpuscles are shown within the heart 

 in figure 75. 



Since the organs and tissues which form the subject of this 

 section are all derived from the mesoderm, a consideration of the 

 development of this germ layer naturally comes next in order. 

 The development of the mesoderm in the head, including the 

 gnathal segments, differs from that of the trunk, and since the 

 latter may be regarded as representing a less modified and there- 

 fore more typical condition, it will be considered first. 



