DEVELOPMENT OF PHRTGANIDS. 583 



to Rathke and Zaddach, and Brandt among recent authors, 

 this pointed organ serves to rupture the egg membrane — which 

 I doubt very much, the only evidence of that purpose being in 

 its apparent adaptability — its chief function probably does not 

 lie in that alone, being merely a secondary adaptation of its 

 primary function as a sense organ. 



The compound eyes appear, just before the revolution of the 

 embryo on the surface of the procephalic lobes, as round re- 

 fractive areas, in which subsequently six dark red pigment spots 

 appear (PI. XXXVIa, figs. 13—16). A section through the 

 eye at this period shows a thickened area of ectoderm, which 

 ultimately becomes connected with the brain (PL XXXVIb, 

 figs. 15 and 19). 



Tracheae are formed in all the post-oral segments, with the 

 exception of the last two or three abdominal. The tracheal 

 invaginations are less conspicuous in the abdominal segments 

 than elsewhere; they are formed by an invagination of the 

 ectoderm close to the nervous ganglia (PL XXXVIb, fig. 20, /). 

 In the thorax and head the invaginations occur on the outer or 

 dorsal sides of the appendages, the tracheae thus formed lose 

 their connection with the exterior, and, increasing in length in 

 an antero-posterior direction, fuse with each other and form 

 common tracheal trunks one on each side of the body. 



The spinning glands are formed by a pair of ectodermic in- 

 vaginations on the ventral side of the embryo, between the 

 base of the second maxillae and the nervous cord. They 

 increase rapidly in length until they are nearly two thirds as 

 long as the embryo, and when the second maxillae fuse they 

 also unite to form a common duct, which opens at the end of 

 the upper lip. 



The salivary glands are formed by invagination of the ecto- 

 derm on the inner sides of the mandibles, in the same manner 

 as are the spinning glands. They are short tubes reaching 

 back behind the second maxillae. 



The mesoderm in the preceding stage had separated into 

 segments, the lateral extremities of which were formed of two 

 irregular layers of cells ; no definite body cavity had appeared. 



