THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 7 



series of somites. The somites thus formed are solid. Figure VIII' is a diagram- 

 matic drawing of a longitudinal section of an embryo at this stage of development. 

 The somites of the inner layer are seen in longitudinal section (DS, in Fig. VIII). 

 It will also be seen that the body somites are marked by incisions of the outer layer 

 or ectoderm extending between the somites of the inner layer. The inner layer ex- 

 tends the entire length of the embryo. 



The dotted line joining the ends of Fig. VIII' represents the amnion in longi- 

 tudinal section. The extreme ends of the embryo, in referring to figure VIII', are 

 seen to be curved dorsally, that is, away from the amniotic side of the embryo. The 

 two terminal somites thus have their body wall formed dorsally before the dorsal 

 surface of the intermediate portion of the embryo is closed. 



At a later stage of development the ectoderm becomes thickened on each side 

 of the blastopore ; consequently there are formed two thickened strings of ectoderm 

 which are the first indications of the nervous system. Figure XI is a drawing of a 

 transverse section of an embryo at this stage. NS in Figure XI represents the 

 thickened ectoderm lying on each side of the blastopore (BH in Fig.). 



Subsequently the thickened ectoderm strings which are to form the nerve cords 

 become separated from the superficial ectoderm, and also from the ectoderm covering 

 the sides and bottom of the blastopore. The ectoderm cells at the bottom of the 

 grove or blastopore (BH, Fig. XII) become greatly elongated. These elongated 

 cells lie close to the nerve cords, but do not actually join them. Figure XII, repre- 

 sents a transverse section of an embryo at this stage of development. The nerve 

 cords (NS in Fig.) will be seen to be separated from the surface ectoderm lining the 

 groove. The elongated ectoderm or what might more properly be called indifferent 

 cells at the bottom of the groove will also be observed in the figure. When the 

 nerve cords have been separated from the surface ectoderm they segment; the num- 

 ber of somites corresponding to the number of body segments. , 



Hatscheck, (1) from his studies on the Lepidoptera, thought the elongated ecto- 

 derm or indifferent cells lying at the bottom of the blastopore, ultimately fused with 

 the nerve ganglia and formed the long commissure of the nervous system. 



In this, as subsequent stages in the development of Thyridopteryx show, he 



(i) Beitrage. Zur Entwicklung d. Lepidoptera. Jenaische Zeitschrift Bd XI. 



