ECTODERMAL DERIVATIVES 97 



longitudinal trunks are well developed in the embryo, although the 

 system is a closed one. Tiegs and Murray (1938) found a total of 10 

 pairs of spiracles in the embryo of Calandra oryzae of which all but the 

 first and last pairs later close, a condition normally occurring in the 

 embrj^os of the muscoidean flies also. 



OENOCYTES 



Certain large cells found in the body cavities of the pterygote insects 

 and usually associated mth the fat cells are the oenocytes. They 

 originate segmentally in the embryo from the ectoderm at points just 

 behind the spiracles of the first eight segments. Thej^ have also been 

 observed in the thorax into which they have probably been crowded from 

 the first abdominal segment. Heymons (1895a) has also found them in 

 the eleventh abdominal segment of Forficula where they appear after 

 blastokinesis at the time those in the spiracle-bearing segments have 

 come to lie in the fat body as globular groups of cells. Roonwal (1937) 

 states that in the late embryos of Locusta migratoria the oenocytes extend 

 into the ninth and tenth abdominal segments, probably as extensions 

 from the eighth. In these last two segments they do not exhibit a 

 metameric arrangement. 



THE NERVOUS SYSTEM 



The Ventral Nerve Cord. — Shortly after the separation of the inner 

 layer from the ectoderm a median longitudinal neural groove usually 

 forms on the ventral side of the embryo. To the right and the left of 

 this groove, cells, characterized by their larger size, have differentiated 

 from the ectoderm and, sinking below the surface, form the neuroblasts, 

 or primary nerve cells. The presence of these rows of neuroblasts in 

 the form of lateral cords is marked on the embryo by longitudinal thick- 

 enings, or neural ridges, one on each side of the groove. The neural 

 ridges thus each have a layer of smaller epidermal (dermatogene) cells 

 outwardly and a longitudinal strip of neuroblasts, a few cells in width, 

 inwardly (Fig. 45). Active division of the neuroblasts now takes place; 

 the repeated division of the neuroblasts in about the same plane results 

 in a column of daughter cells more or less perpendicular to the ectoderm 

 (Fig. 46). These daughter cells are the future ganglion cells. Between 

 the neural ridges, dorsad of the neural groove, a median cord is formed 

 by invagination of the cells and, hke the lateral cords, is separated as a 

 neurogene strip from the dermatogene layer. In each interganglionic 

 region, or perhaps in the posterior part of the ganglionic region of the 

 median cord {mst), one or more neuroblasts develop from which smaller 

 daughter cells arise as in the lateral cords. When the germ band under- 

 goes segmentation, the lateral nerve cords likewise are metamerically 



