128 



MESOBLAST AND NOTOCHORD. 



The mesoblast and notochord : Anura. After the disap- 

 pearance of the segmentation cavity, the mesoblast is described 

 by most observers, including Gotte, as forming a continuous 

 sheet round the ovum, underneath the epiblast. The first 

 important differentiations in it take place, as in the case of the 

 epiblast, in the axial dorsal line. Along this line a central cord 

 of the mesoblast becomes separated from the two lateral sheets 

 to form the notochord. Calberla states, however, that when the 

 mesoblast is distinctly separated from the hypoblast it does not 

 form a continuous sheet, but two sheets one on each side, 

 between which is placed a ridge of cells continuous with the 

 hypoblastic sheet. This ridge subsequently becomes separated 

 from the hypoblast as the notochord. Against this view Gotte 

 has recently strongly protested, and given a series of careful 

 representations of his sections which certainly support his 

 original account. 



My own observations are in fa- 

 vour of Calberla's statement, and 

 so far as I can determine from my 

 sections the mesoblast never ap- 

 pears as a perfectly continuous 

 sheet, but is always deficient in the 

 dorsal median line. My observa- 

 tions are unfortunately not found- 

 ed on a sufficient series of sections 

 to settle the point definitely. 



After the formation of the 

 notochord (fig. 72), the meso- 

 blast may be regarded as con- 

 sisting of two lateral plates, 

 continuous ventrally, but sepa- 

 rated in the median dorsal 

 line. By the division of the 

 dorsal parts of these plates 

 into segments, which com- 

 mences in the region of the 

 neck and thence extends back- 

 wards, the mesoblast of the 

 trunk becomes divided into 



FIG. 74. SECTION THROUGH THE AN- 

 TERIOR PART OF THE TRUNK OF A YOUNG 

 EMBRYO OF BOMBINATOR. (After Gotte.) 



as" . medulla oblongata ; is*, splanchno- 

 pleure; as*, somatopleure in the vertebral 

 part of the mesoblastic plate ; s. lateral plate 

 of mesoblast ; f. throat ; e. passage of epi- 

 thelial cells into yolk-cells ; d. yolk-cells ; 

 r. dorsal groove along the line of junction of 

 the medullary folds. 



