VIII 



ARTHROPODA 



265 



embryonic area, and then segmentation of the embryonic area occurs ; 

 there is, however, no gastral grooye. An intercalary segment is 

 clearly indicated. AU the abdominal segments develop appendages. 

 Of the three anal cerci which the adult possesses, the two lateral 

 represent the appendages of the eleventh abdominal segment, the 

 median a backward prolongation of the telson. The genital cells, as in 

 Donacia, originate from a group situated in the posterior end of the 

 embryonic area; they wander forward and are eventually found in 

 the dorsal section of the somites. 



Tlie most interesting feature in the development of Zepisma 

 concerns the formation of membranes. As the embryonic area 

 becomes segmented it becomes bodily invaginated into the yolk, but 

 the opening of the invagination, which we may term the amniotic 

 pore, is never closed. It follows that the greater part] of the 



B 



Fig. 210. — Two views of the egg Lepis7na saccharina in ditt'erent stages of development. 



(After Heymons.) 

 A, sagittal section of the stage of invagination of tlie embryonic area. B, external view, stage of 

 exsertion of embryo, am, amnion ; am.p, amniotic pore ; em6, embryonic area ; ser, serosa ; s.d.o, 

 secondary dorsal organ ; y, yolk, 



ectoderm covering the egg corresponds to the serosa, and the nuclei 

 of these ectodermal cells undergo the same characteristic modifica- 

 tion as do the nuclei of the serosa cells of Donacia. The lateral 

 walls of the invagination cavity, as they pass down to join the ends 

 of the embryonic area, consist of ordinary flat cells with normal 

 nuclei and correspond to the amnion. 



Turning to another representative of the Aptera, Machilis, which 

 is unique amongst insects in retaining rudimentary appendages on 

 the abdomen throughout life, we find still more primitive conditions. 

 Here, too, the embryonic area becomes invaginated, but the in- 

 vagination is not deep, and a comparatively large opening connects 

 the invagination cavity with the exterior. The front half of the egg 

 alone is covered with ectoderm, whose nuclei undergo the characteristic 

 serosa modification; the hinder half remains covered with cells having 

 ordinary nuclei, and represents the amnion. Heymons suggests 

 that the serosa ectoderm, with its peculiar nuclei, has a distinct 

 physiological function, possibly connected with the transfusion of gas. 



