6 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



comes to lie between the amnion and blastoderm. In the space between the amnion 

 and ventral surface of the embryo, however, there is no yolk. The amnion consists 

 of flat hexagonal cells resembling the blastoderm cells in appearance. Later the 

 embryo lying in the centre of the yolk becomes pear-shaped, with its broad anterior 

 end deeply cleft. The shape of the embryo at this stage will be understood from 

 an examination of Figs. VIII, VIII' & IX. Figure VIII, shows the outline of the 

 embryo at this stage as indicated by a horizontal section. Figure VIII' represents 

 diagrammatically a longitudinal section of an embryo at about the same stage of 

 development. The lateral portions of the broad anterior end of the embryo, sepa- 

 rated by the deep cleft are the so called pro-cephalic lobes (P C Fig. VIII). Figure 

 IX is a less diagrammatic reproduction of a horizontal section of an embryo at the 

 same stage of development as figure VIII. B L, in Figure IX stands for the 

 blastoderm. The space between the blastoderm and embryo in the section, was of 

 course filled with yolk balls, only one of which (Y B in Fig. IX) is drawn. 



The cleft separating the pro-cephalic lobes is a continuation of a median groove 

 extending along the median line of the ventral surface of the embryo at this stage. 

 This groove or depression on the ventral surface of the embryo causes an elevation 

 on its other surface, that is, on its surface towards the future dorsal wall which has 

 not yet been formed. Figure VII represents a highly magnified transverse section 

 of an embryo at a stage when the median groove deepens, beginning to pushdorsally 

 the median portion of the embryo (D in Fig. VII). In subsequent stages the groove 

 deepens, and the pushed in portion of the embryo becomes folded off, and forms the 

 inner layer (i in Figs). The inner layer is not strictly invaginated, for it is cut off 

 from the rest of the embryo before the opposite sides of the median groove have 

 met. After its separation the inner layer splits into two bands. The line of sepa- 

 ration of these two bands being immediately beneath the blastopore. Figure X is 

 a transverse section of an embryo at this stage. The inner layer (I in Fig. X), it 

 will be observed, is divided into bands, the line of separation being a continuation 

 of the median groove (BH in Fig). Figure X' represents the same section as X 

 drawn with a lower power, showing the relation of the embryo to the yolk balls and 

 blastoderm (BL). On its separation into two bands or shortly after the separation 

 has been effected, each band of the inner layer segments or becomes divided into a 



