KM; 



MESOBLAXT AND NOTOCHOKD. 



rated from the hypoblast as the notochord. Against this view Gotte 

 has recently strongly protested, and given a series of careful repre- 

 sentations of his sections which certainly support his original ac- 

 count. 



My own observations are in favour of Calberla's statement, and so far 

 as I can determine from my sections the mesoblast never appears as a 

 perfectly continuous sheet, but is always deficient in the dorsal median 

 line. My observations are unfortunately not founded on a sufficient series 

 of sections to settle the point definitely. 



After the formation of the notochord (fig. 72), the mesoblast may 

 be regarded as consisting of two lateral plates, continuous ventrally, 

 but separated in the median dorsal line. By the division of the dorsal 

 parts of these plates into segments, which commences in the region 

 of the neck and thence extends backwards, the mesoblast of the 



trunk becomes divided into a 

 vertebral portion, cleft into se- 

 parate somites, and a lateral un- 

 segmented portion (fig. 74). 



The history of these two parts 

 and of the mesoblast is generally 

 the same as in Elasmobranchs. 



The mesoblast in the head be- 

 comes, according to Gb'tte, divided 

 into four segments, equivalent to 

 the trunk somites. Owing to a 

 confusion into which Gotte has fallen 

 from not recognizing the epiblastic 

 origin of the cranial nerves, his 

 statements on. this head must, I 

 think, be accepted with considerable 

 reserve ; but some part of his seg- 

 ments appears to correspond with 

 the head-cavities of Elasmobraiichii. 



Urodela. Scott and Usborn 

 (No. 114) have shewn that in the 

 Newt the mesoblast (tig. 75) is 

 formed of two lateral plates, split 



FIG. 74. SECTION THROUGH THE AN- 

 TKRIOK PART OF THE TRDNK OF A YOUNG 

 EMBRYO OF BoMBiNATOR. (After Gotte. ) 



as'", medulla oblongata; iV. splanchno- off from tbe hypoblast, and that 

 pleure; s*. somatopleure in the vertebral the ventral growth of these plates 

 1'iirt of the mesoblastic plate; s. lateral 

 plate of mesoblast ; /. throat; e, passage of 

 epithelial cells into yolk-cells; d. yolk-cells; 

 i: dorsal groove along the Hue of junction 

 of tbe medullary folds. 



is largely effected by the conversion 

 of yolk-cells into mesoblast- eel Is. 

 They have further shewn that the 

 notochord is formed of an axial 

 portion of the hypoblast, as in the 

 types already considered (fig. 75). The body-cavity is continued into the 

 region of the head ; and the mesoblast lining the cephalic section of the 

 body-cavity is divided into the same number of head-cavities as in Elasuio- 

 braiichii, viz. one in front of the mouth, and one in the mandibular and 

 one in each of the following arches. 



