EPHEMERIDA, ODONATA, PLECOPTERA, ET AL. 207 



posterior end, with the micropyhir area at the anterior end. Within the 

 egg the usual viteUine membrane, periplasm, protoplasmic reticulum, and 

 food yolk are found. The fifth day after deposition two types of cells may 

 be seen in the yolk, the cleavage cells which later penetrate the periplasm 

 to form the blastoderm, and yolk cells which remain behind in the j^olk. 

 After the formation of the blastoderm the yolk cells actively divide and 

 become scattered. At the posterior end of the egg a clump of sex cells 

 becomes differentiated and enters the yolk. When the elongated germ 

 band has formed on the ventral side, the head lobes retract from the 

 anterior end of the egg as the tail pushes over on the dorsal side carrjdng 

 the sex cells with it. Above the gastrular furrow the cells of the inner 

 layer are formed, with the amnion and serosa developing in a normal 

 manner. Segmentation of head and thorax takes place before that of the 

 abdomen, and the appendages of the eleventh abdominal segment appear 

 before the rudimentary appendages of the intermediate abdominal seg- 

 ments. Mesoderm segmentation occurs simultaneously with, that of the 

 body wall. Development of the ventral nerve cord offers nothing unusual. 

 Heymons failed to find a ganglion in the eleventh abdominal segment. 



In the formation of the compound eye an inner laj'er of ectodermal 

 cells gives rise to the optic ganglion which does not entirely lose its connec- 

 tion with the outer ectodermic layer that wdll form the eye plate. There 

 is, therefore, a direct nerve connection from the beginning and not a 

 subsequent union, as stated by Viallanes and Wheeler for the insects 

 studied by them. 



Laterally coelomic sacs develop in the mesoderm extending into the 

 appendages. No sacs form in the short preantennal section. In the 

 premandibular region, between the antennal and mandibular segments, 

 the lateral mesoderm is two-layered, perhaps indicating rudimentary 

 coelomic sacs. The walls of the coelomic cavity in the antennal segment, 

 which is most highly developed, become very thin. The sacs in the 

 gnathal and thoracic segments are smaller, and none forms in the eleventh 

 segment. The dorsolateral part of the coelomic sacs represents the 

 pericardial septum, which later develops into the segmentally arranged 

 "wing muscles" of the heart, uniting below the heart tube as the dorsal 

 diaphragm. Mesad of these cells groups of cells distinguished by their 

 paler color and elongate form are found in each segment which together 

 form the longitudinal paracardial cell strands that eventually come to he 

 on each side of the heart in the walls of the pericardial septum. Since the 

 pericardial septum retracts from the epidermis, a pericardial chamber is 

 formed in which a fat body develops. The fat body is formed from the 

 cells at the junction of the dorsal and ventral sections of the somatic wall. 

 Later, distinct in postembryonic life, pericardial cells also develop in 

 addition to the paracardial cell strand. They are probably derived from 

 middle section of the pericardial septum. 



