48 J. F. MCCLENDON. 



support his view, comparing the process to a re-arrangement of 

 pairs of neuromeres. But the parts that are displaced in the 

 process are small masses of nerve tissue without nerves or cross- 

 commissures and could not be regarded as neuromeres unless 

 associated with separate segments of the body. This, Patten 

 attempted to do, supposing the segments of all arthropods to be 

 double, as shown by the frequent presence of " bifurcated 

 appendages " and the " frequent occurrence of insect monsters 

 having double pairs of legs." On the contrary my observations 

 and reading lead me to believe that, with the exception of the 

 diplopods (Jnlits), it is probable that the segments of all arthro- 

 pods are single. In the scorpion embryo each segment except 

 the first has one pair of mesoblastic somites and one pair of 

 appendages. 



The semilunar lobe (si, indicated by a transverse band shaded 

 slightly darker) has elongated somewhat and has moved up- 

 ward and backward with the flexure of the anterior part of the 

 neural band. In Figs. 5, 6, 7, 8 and 9 this process may be seen 

 to continue until the anterior part of the nerve band is first per- 

 pendicular to the posterior or subenteric portion, and then by 

 continued flexure bends backward. The ectoderm continues to 

 grow over the first neuromere (<r<;). The mouth has continued 

 to move backward, bending the oesophagus in the form of an arc 

 (ce), compare Fig. 8, a\ 



The appendages of the tenth segment are elongating to form 

 the pectines, while those of segments eleven to fourteen have dis- 

 appeared, their place being taken by lung books. Sterno-coxal 

 processes, called maxillaria by Patten, have appeared on the fourth 

 and fifth segments (////r). 



Stage 5. -- (Fig. 5.) In this stage the pits have almost entirely 

 disappeared from the nervous tissue, being filled up by the growth 

 of the cells composing their walls. The ectoderm has grown 

 over the ventral chain, which has now become more compact. 

 The nerves have begun to assume their definite form (4^, ^P], 

 that is to say the peripheral nerve fibers are being compacted into 

 nerves that can be distinguished in sections and sometimes in 

 surface views. The ectoderm has grown further back over the 

 head (ec). Pigment is beginning to appear in the median eyes. 



