12 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



Figures XXIX' and XXX' Plate II are diagrammatic representations of the 

 head viewed from its under surface and laterally. The upper lip is seen to be dis- 

 tinctly bilobed. The first maxillae are longer than the other cephalic appendages. 



The sense organs are what may be termed compound simple eyes. They are 

 represented in figures XXVII and XXVIII. Each ocellus, (OC in figures,) appar- 

 ently arises as an involution of cells from the inner surface of the ectoderm. These 

 involutions form sacks apparently containing no cells ; but the lumen or cavity of 

 the sack is filled with granular material. Migratory mesoderm cells probably in- 

 vest these ocelli. 



It does not appear that any part of the ocelli is formed from the nervous 

 system. The observations which were made were not conclusive on this point 

 however. 



An invagination (GL, Figs. XXVII & XXVIII) occurs at the base of the 

 mandible which apparently forms the salivary gland. 



It remains now to describe the origin of the endoderm and the closure of the 

 dorsal surface of the body. The appendages and tracheal invaginations arise in the 

 customary manner. 



A portion of the inner layer on each side of the embryo becomes separated 

 from other parts of the inner layer (IE Figs. XV, XXIV, XXV.) 



These portions of the inner layer which may be called endoderm grow together 

 and unite first on what is the ventral surface of the alimentary tract (Fig. XXV.) 

 They then extend dorsally and enclose the yolk and the yolk cells which lie in the 

 body cavity. 



Figure XXXII is a cross section, already explained, of an advanced embryo in 

 which the epithelium of the midgut is fully formed. The yolk cells lie, with the 

 yolk, in the digestive tract and certainly do not form any considerable portion, if 

 any, of the endoderm. A similar formation of endoderm has been described by 

 Tichomiroff l for other Lepidoptera. His conclusions have been disputed, notably 

 by Balfour, who claims that the yolk cells are the true endoderm. Before its 

 closure by the endoderm, the yolk is enclosed by migratory mesoderm cells (I 1 

 Fig. XXIV.) Inside these cells the endoderm grows round and encloses the yolk. 



(') Zool. Anzeiger. II Jahr. no. 20. 



