No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 91 



tion and it seems to me that they can have only two possible 

 sources — they either arise from some of the progeny of the 

 neuroblasts or from the intraganglionic portion of the median 

 cord. I deem it highly improbable that they should arise from 

 the former source, since the daughter-cells of the neuroblasts 

 have every appearance of being early specialized as ganglion 

 cells. Furthermore, the cells of the neurilemmata when they 

 definitely appear, closely resemble the cells of the neural fur- 

 row both in size and in the great avidity with which they 

 take the stain. The outer neurilemma covers first the inner 

 surface of the ganglion — then the outer or neuroblastic sur- 

 face; — the thin cellular membrane apparently progressing 

 ing laterad in either case and meeting near the origin of the 

 nerve-trunks. The inner neurilemma, which envelops the 

 Punktsubstanz is completed before the outer envelope. Histo- 

 logically both envelopes resemble each other in every respect. 

 The fusions of ganglia in the nerve-cord take place gradually 

 and may be easily followed in XipJiidium. Several stages of 

 these fusions are represented in Fig. VII, A-D. In Fig. A, 

 the nerve-cord is shown much as it appears in Stage F. The 

 ganglia form an unbroken series from mouth to anus. The 

 connectives are very short and not as yet distinguishable from 

 the surface. Fig. B, is taken from an embryo just turning the 

 lower pole (Stage H). Here the mandibular and two maxillary 

 ganglia, and also the three terminal abdominal ganglia still 

 remain as in the preceding stage, while the other ganglia are 

 being drawn apart by the stretching of the embryo, so as to 

 show their short connectives. In Fig. C, the suboesophageal 

 and last abdominal ganglia are established as two fused masses. 

 The number of ganglia comprising each of these masses may 

 still be easily determined by counting the commissures. It 

 will be noticed that in this stage the first abdominal is closely 

 approximated to the metathoracic ganglion and that the second 

 and third abdominal also lie close together. Between the 

 other ganglia the connectives have lengthened. In the later 

 stage represented in Fig. D, the connectives are still longer; 

 the first abdominal ganglion has fused with the metathoracic, 

 and th© second and third abdominal form a single mass. 



