THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 3 



yolk spherules. The spherules (YS in Fig.) are frequently vacctiolated. No trace 

 of a germinal spot or vesicle was found in the mature egg. The former might 

 readily have been present and have escaped observation. 



The early stages of segmentation have already been studied in the Lepidoptera 

 by Bobretzky.'" In the earliest stages he found four amoebiform cells in the yolk 

 situated in pairs at opposite ends of the egg. Later the blastoderm is formed by a 

 multiplication of these cells: according to Bobretzky it is not' formed simultaneously 

 over the entire surface of the egg, but is laid down first at one or more points on 

 the surface. This type of segmentation cannot strictly be called entolecithal, in as 

 much as the cells are not, in the earliest stages of segmentation, at the surface en- 

 closing the yolk. All the primitive undifferentiated cells do not, according to 

 Bobretzky, reach the surface to form the blastoderm, but some remain centrally lo- 

 cated as 'yolk cells after the formation of the blastoderm. The earliest stages of 

 segmentation observed in Thyridopteryx showed several amoebiform cells in the 

 yolk in each cross section. 



The egg at this stage must consequently contain a number of cells most of 

 which have not reached the surface. The nuclei of these centrally located cells 

 generally have indistinct boundaries which become more distinct and their contents 

 clearer on reaching the svirface (Fig. III')-. Figure III' represents a cell at the sur- 

 face undergoing division. It has two nuclei with clear contents and well defined 

 boundaries. 



The other cell represented in the figure has just reached the surface of the egg. 

 Its nucleus is indistinct and granular. Much reliance cannot be placed on histologi- 

 cal details like these, however, except in case of remarkably well preserved specimens. 



In Thyridopteryx, as in the Lepidopterous insects studied by Bobretzky-j the 

 blastoderm does not appear simultaneously over the entire surface of the egg but 

 appears first at one point on the surface. Figure III is a transverse section of the 

 egg at this stage. The yolk spherules are not represented in the figure. It will 

 be observed that the blastoderm is only formed over a portion of the surface. 



Closely related insects are said to differ in their blastoderm formation, however, 

 and the simultaneous or unsimultaneous appearance of the blastoderm over 



(') Bobretzky. Uber die Bildung des Blastoderms und den Keiniblntter bei dt-n Insecten. 



