20 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



neurp mst 



eci neurg neur 



Fig. 22. — Section of ventral nerve 

 cord, {dt) Daughter cells, {ect) Ecto- 

 derm, {mat) Median nerve strand. 

 {neur) Neuroblast, {neurg) Neural 

 groove, {neurp) Neuropile. {From 

 Wheeler.) 



entirety, therefore, first develops at each extremity and later is formed 

 in the middle. 



As has already been stated, blood cells arise from cells liberated along 

 the midventral line above the inner neural ridge. By some embryologists 



these are regarded as coming from 

 the inner lateral margins of the 

 coelomic sacs adjacent to the middle 

 strand and therefore mesodermic; 

 other writers, Hirschler (1909) 

 among them, say that these cells 

 come from the middle strand itself 

 and therefore, hke the mid-gut 

 epithelium, are entodermic. 



The pericardial cells, likewise re- 

 garded by some embryologists as 

 mesodermic and probably derived 

 from the middle section of the peri- 

 cardial septum (Fig. 21, yd), are said 

 by Hirschler (1909) to arise from 

 those cells of the posterior mesen- 

 teron rudiment which are not used in the formation of the posterior 

 part of the mid-gut epithelium. 



The paired structure found in the pericardial septum in metameric 

 arrangement at the sides of the 

 dorsal blood vessel in Forficula 

 and some other insects, and known 

 as the paracardial cell strand, 

 arises from the dorsal part of the 

 coelomic sacs. 



Nervous System. — Shortly 

 before the appearance of the 

 stomodaeal and proctodaeal invag- 

 inations the neural groove (Fig. 

 16, neurg) forms along the mid- 

 ventral longitudinal line, extend- 

 ing at a little later stage from near 

 the stomodaeum to the procto- 

 daeum. On each side of the 

 groove outwardly is an outer 

 neural ridge which later develops into the paired nerve strands. The 

 outer neural ridges become segmented. The ectoderm adjacent to the 

 neural groove becomes two-layered, the outer layer consisting of ordinary 

 body-wall cells, the inner layer composed of large nerve cells, or 



Fig. 23. — Cross section of posterior end 

 of an early embryo. Germ cells {gc) lodged 

 between the primary epithelium and the yolk. 

 {From Hirschler.) 



