THE SOMITES OF THE CHICK 83 



upward laterally and, as the disintegration of the cutis plate has 

 proceeded but slowly, the upper part of the area of partial 

 disintegration is shown in the section. The spinal ganglion is 

 now well defined and is quite surrounded by mesenchyma. The 

 spinal nerve extends downward nearly into the section figured. 

 The sclerotome is bounded anteriorly and posteriorly by the in- 

 tersegmental vessels {I. A., I.V.), laterally by the muscle plate 

 (M.), medially by the notochord (A'"), and at a higher level by the 

 neural tube, ventrally by the aorta and the posterior cardinal vein. 

 The sheet of sclerotomic tissue between the aorta and the noto- 

 chord has become considerably thicker, and a similar but very thin 

 sheet separates the notochord from the neural tube. The loose 

 tissue surrounding the notochord, and extending as far laterally as 

 the intersegmental arteries (I. A.), forms a continuous perichordal 

 sheath in which no visible condensations occur. The interseg- 

 mental arteries mark the outer limit of the future vertebral cen- 

 tra and the body of each vertebra will form in this originally ho- 

 mogeneous perichordal sheath which was first described by Gegen- 

 baur. No portion of this sheath can be assigned with accuracy to 

 any particular somite or sclerotome, for we have seen that the an- 

 terior ends of the sclerotomes of the second to the fifteenth 

 segments extend far forward under the preceding sclerotomes. 

 Laterally to the intersegmental arteries the sclerotomic tissue 

 rapidly becomes denser except in the middle of the sclerotome 

 where a thin vertical zone of loose tissue, the "intervertebral 

 fissure," divides the denser tissue into distinct anterior and 

 posterior portions which take the form of square columns. These 

 two columns of dense sclerotomic tissue are generally, but I 

 believe wrongly, regarded as morphological entities. There is, 

 however, some difference of opinion as to their exact meaning. 

 Those who hold to Remak's theory of vertebral resegmentation as 

 modified by Von Ebner, consider them to be the anterior and pos- 

 terior halves of the right or left half of the ''primitive vertebra." 

 Others, however, who accept Schauinsland's theory (which, as was 

 pointed out above, is closely related to Goette's theory) that there 

 are two ''primitive vertebrae" in each segment, regard each col- 

 umn of dense sclerotomic tissue as simply one-half of the anterior 



