THE MIDDLE CORD OF THE SCORPION. 



329 



The outer layer is denser, contains a few small dark nuclei and scattering 

 bundles of longitudinal and circular fibers; it appears to be continuous with the 

 inner fibrous layer of the neurilemma. The sinus and the whole spinal cord is 

 surrounded by a layer of large granular cells which vary greatly in number and 

 arrangement in different parts of the body. 



The Bothroidal Cord or Lemmatochord. Along the haemal surface of the neural 

 sinus is an elongated, lobular organ extending the whole length of the abdomen. 

 At irregular intervals it forms large, spindle-shaped bothroidal masses that are 

 united with each other by a very delicate hyaline fiber. (Figs. 71, 72). The masses 

 vary in number and size; in three different specimens I have counted 4, 7, and 9 

 of them. In sections they have a lymphoid appearance, and are seen to consist of 

 dense, indistinctly fibrous masses crowded with minute, deeply stained nuclei. 

 (Fig. 22$,l.ch.) They are united here and there with the neural sinus by short 

 stalks. In one specimen there were ten attachments of the cord to the sinus, six 

 of which were hollow and opened into the neural sinus. 



In the embryos the tissue from which the bothroidal cord arises extends 

 forward into the thorax, where it forms segmental thickenings between the succes- 

 sive neuromeres. All these thoracic thickenings disappear, with the exception of 

 the one beneath the connectives of the fifth and sixth thoracic neuromeres. This 

 one becomes the merochord. It lies on the neural surface of the endocranium 

 near the anterior edge of the cross bar. (Fig. 71.) The adult merochord (Fig. 

 225 5 ) is a large rounded body containing a few clear cells, many small dark nuclei, 

 and irregularly coiled muscle strands. 



The overlying interganglionic space is closed and is filled with a darkcoagulum 

 connected with the merochord by an irregular reticulum. 



B. Development of the Lemmatochord. The lemmatochord arises, in 

 part, as an axial cord of cells extending forward from the primitive streak. In 

 stage A, Fig. 15, the primitive streak is seen as a large median mass of 

 polygonal cells near the posterior end of the embryo. In sections the cord appears 

 to form as an inward proliferation of the surface cells, but without the surface 

 infolding seen in Limulus. (Fig. 226 1 .) From the point of proliferation, covering 

 but one or two sections, the cord extends forward a short distance as a well de- 

 fined cylinder. In stage B, it is greatly reduced in thickness and forms a broad 

 lenticular band with the edges thinned out to a single layer of cells, not sharply 

 marked off from either mesoderm or endoderm. It is largest just in front of the 

 base of the tail lobe, and extends forward, becoming less and less distinct, as far 

 as the third abdominal neuromere. It does not extend into the tail lobe. 



In the following stages, up to stage G, the primitive streak forms lens-shaped 

 thickenings beneath the 3, 4, 5, 6, and 7 interganglionic spaces of the abdomen. 

 It is easy enough to distinguish these thickenings at this stage, but difficult to 

 determine their lateral boundaries. A fairly defined layer of flattened cells 

 separates them from the yolk. (Fig. 228.) 



In stage K, Fig. 229, the primitive streak has definitely split along its whole 



