TRACHEATA. 



385 



formed as cuticular investments of papillae in pockets of the 

 skin at the ends of their terminal joints. 



I have been able to make a few observations on the internal structure of 

 the embryos from specimens supplied to me by Moseley. These are so far 

 confined to a few stages, one slightly earlier, the others slightly later, than 

 the embryo represented in fig. 168 B. The epiblast is formed of a layer of 

 columnar cells, two deep on the ventral surface, except along the median line 

 where there is a well-marked groove and the epiblast is much thinner (fig. 172). 



The ventral cords of the trunk are formed as two independent epiblastic 

 thickenings. In my earlier stage these are barely separated from the 

 epiblast, but in the later ones are quite independent (fig. 172, v.n), and 

 partly surrounded by mesoblast. 



The supra-cesophageal ganglia are formed as thickenings of the epiblast 

 of the ventral side of the procephalic lobes in front of the stomodaeum. 

 They are shewn at / in fig. 170. The thickenings of the two sides are at 

 first independent. At a somewhat later period an invagination of the 

 epiblast grows into each of these lobes. The openings of these invaginations 

 extend from the oral cavity forwards; and they are shewn in fig. 171 l . 

 Their openings become closed, and the walls of the invaginations constitute 

 a large part of the embryonic supra-cesophageal ganglia. 



Similar epiblastic invaginations assist in forming the supra-cesophageal 

 ganglia of other Tracheata. 

 They are described in the sequel 

 for Insects, Spiders and Scor- 

 pions. The position of the supra- 

 cesophageal ganglia on the ven- 

 tral side of the procephalic lobes 

 is the same as that in other 

 Tracheata. 



The mesoblast is formed, in 

 the earliest of my embryos, of 

 scattered cells in the fairly wide 

 space between the mesenteron 

 and the epiblast. There are two 

 distinct bands of mesoblast on 

 the outer sides of the nervous 

 cords. In the later stage the 

 mesoblast is divided into dis- 

 tinct somatic and splanchnic lay- 

 ers, both very thin ; but the two 

 layers are connected by trans- 

 verse strands (fig. 172). There 



sp.w 



$.m 



FIG. 172. SECTION THROUGH THE TRUNK 

 OF AN EMBRYO OF PERIPATUS. The embryo 

 from which the section is taken was somewhat 

 younger than fig. 171. 



sp.m. splanchnic mesoblast. 



s.m. somatic mesoblast. 



me. median section of body cavity. 



k. lateral section of body cavity. 



v.n. ventral nerve cord. 



me. mesenteron. 



1 This figure is taken from Moseley. The epiblastic invaginations are represented 

 in it very accurately, and though not mentioned in the text of the paper, Moseley 

 informs me that he has long been aware of the homology of these folds with those in 

 various other Tracheata. 



B. II. 



2 5 



