4 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



the entire surface is consequently a fact of little erabryological importance. 



The first trace of the embryo is a thickening of the blastoderm at one point 

 where it becomes more than one cell thick (Figs. IV & IV). Whether this thick- 

 ening results from the division in a single layered blastoderm at that point, or from 

 a special aggregate of embryonic cells occurring there, could not be satisfactorily 

 determined. Figure IV represents a transverse section of an egg showing the early 

 embryo or ventral plate, E in fig. in cross section. Figure IV is a drawing of the 

 embryo at this stage more highly magnified. On either side of the embryo, as seen 

 in cross section, are seen folds of blastoderm (Am F. Fig. IV) which ultimately 

 grow together and unite over the median line of the embryo. On the union of these 

 folds, the embryo with the inner limb of the fold or true amnion becomes separated 

 from the outer fold or serosa, and comes to lie in the yolk (Fig. V), while the serosa 

 remains continuous with the blastoderm. At the stage represented by Figure IV 

 it should be noted that some cells have not reached the surface, but remain in the 

 yolk enclosed by the blastoderm (YC Fig. IV). These are the so called yolk cells. 

 Concerning the origin and fate of these cells in different insect groups there is con- 

 siderable difference of opinion among investigators. Some investigators, notably 

 Balfour, in his work on Embryology, from evidence afforded by Bobretzky's work on 

 the L/epidoptera just referred to, and from other observations, claims that these 

 central yolk cells are in all cases undifferentiated cells which have never reached the 

 surface, and homologizes them with the endoderm of Arachnids. On the other hand, 

 Patten (1) and probably Graber ^ believe that a stage occurs in the development of 

 the insect egg in which there is a blastoderm surrounding a central yolk-mass con- 

 taining no cells. 



The yolk cells which are present in the yolk at subsequent stages must migrate 

 from the blastoderm if this view be correct. Probably both views are correct. In 

 the Lepidoptera from Bobretzky's observations, and from the study of Thyridopteryx, 

 there seems to be little doubt that the first view is true. But from a study of some 

 other groups of insects it seems probable that the second view is also true of those 

 groups. 



To return to the embryo, which it will be remembered had been separated from 



(i) Patten. Development of Phrynganids. Quarterly Journal of Microscopical Science, 1885. 

 (i> Graber. Archiv. f. Mikr. Anat. Vol. XV. 



