242 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



on either side of the median Hne of the ectoderm and facing the yolk. 

 Simultaneously with the multiplication of the neuroblasts the germ-band 

 ectoderm expands on either side of the mid-ventral line to form a pair of 

 neural swellings (Fig. 165, neur). The neuroblasts divide mitotically 

 and give rise to smaller daughter cells (n) which arrange themselves 

 on the inner (dorsal) side of the neuroblasts to form the future ganghon 

 cells (Fig. 165). At about the 112-hour stage the neuroblasts of the 

 median cord (mst) at first lying intersegmentally shift forward and become 

 incorporated into the ganglion of the preceding segment. The ganglion 

 cells send out long processes that develop into the intersegmental pair of 

 connectives and the transverse commissures of each ganglion. A thin 

 layer of elongated cells, the neurilemma, probably arising from the 

 msi " 



neurg neur 



Fig. 165. — Locusta. Cross section of second thoracic segment of 75- to 80-hour embryo. 

 (mst) Median nerve cord, (n) Daughter nerve cells, (neur) Neuroblasts, {neurg) Neural 

 groove. 



outlying ganglion cells, covers the neurogenic tissue dorsally. The total 

 number of nerve ganglia belonging to the ventral chain is 17, one for each 

 segment from the mandibular to the eleventh abdominal segments, 

 inclusive, the last being very small. 



The brain, which consists of the proto-, deuto-, and tritocerebrum, 

 arises, except for the optic ganghon, in a manner similar to that described 

 for the ventral chain of ganglia. The neuroblasts are at first differentiated 

 in the 59-hour stage in the protocerebral part and slightly later in the 

 following parts of the brain. The protocerebral rudiments occupy 

 the entire head lobes. From the beginning the neuroblasts of each half 

 of the protocerebrum are divided into two lobes. A third, the optic, 

 lobe is then differentiated from the lateral ectoderm in a manner different 

 from and independent of the other two lobes but later is connected with 

 them. Neuroblasts do not take part in the formation of the optic lobes. 

 On the dorsolateral edge of the head lobes there appear in the 52-hour 

 stage thickened masses of which the peripheral nuclei are arranged in a 



