50 j. F. MCCLENDON. 



ten, "external" of Police) and a posterior pair (" neural " of 

 Patten, "internal" of Police). In Fig. 6 the anterior nerves are 

 shown in neuromeres four to thirteen (4-13, A) and the posterior 

 nerves in neuromeres three to seven and nine to thirteen (3-7, 

 9-1 3, P}. In the eighth segment the appendages have disappeared, 

 and this may be the reason for the absence of the posterior nerves 

 ot this segment, which would normally innervate these append- 

 ages. The first neuromere has a pair of nerves that innervate 

 the median eyes and a pair of nerves that innervate the lateral 

 eyes. The second neuromere has a pair of nerves that innervate 

 the chelicera and a median unpaired nerve that passes along the 

 oesophagus to the rostrum. This rostral nerve may be a fusion 

 of the anterior nerves of the second neuromere. The cheliceral 

 nerves are serially homologous with the posterior nerves of the 

 segments bearing walking legs. For the same reason I have 

 called the single pair of nerves of the third neuromere posterior 

 nerves. The twentieth neuromere is being drawn up into the 

 nineteenth segment (n 20). 



Stage 7. - - (Figs. 7 and 8. ) By a comparison of Fig. 7, which 

 represents the nervous system as more or less transparent, with 

 Fig. 8, which represents a median sagittal section of the nervous 

 system with an outline of the ectoderm, I hope the reader may 

 get a general idea of this stage of the embryo without the neces- 

 sity of reading much description, and I will call attention only to 

 special points. 



The median-eye sacs have come together in the mid-line and 

 open by a common neck (onto} to the exterior. Nerve-centers 

 nineteen and twenty have completely fused. In the first neuro- 

 mere there are two cross commissures, a fact which leads many to 

 suppose that we have here two neuromeres, as does also the fact 

 that it contains two pairs of optic lobes and two pairs of optic 

 nerves. As to the optic lobes : Saint-Remy has shown that they 

 are intimately associated by their nerve tracts. The significance 

 of the optic nerves in relation to the segmentation cannot be 

 determined here, as they cannot be homologized with anterior or 

 posterior nerves of succeeding segments, for, on the one hand, it 

 is doubtful whether their end organs represent appendages and, 

 on the other hand, their roots cannot be traced to the ventral or 



