514 A. A. W. HUBREOHT. 



The gastrula ridge gives rise to raesoblast in a way that was 

 often described for other mammals and Sauropsida. Cells, 

 evidently in direct connection with the superficial epiblast, 

 proliferate so as to shove in between the primary layers as the 

 lateral plates of peristomal mesoblast. At the posterior end 

 of the gastrula ridge which^ as above remarked^ is so sharply 

 defined in longitudinal sections, mesoblast is similarly seen to 

 proliferate backwards in the direction of the long axis of the 

 embryonic shield, so that the sheet of peristomal mesoblast 

 had better not be compared to two lateral sheets fusing 

 together posteriorly ,i but to a fan-like extension of a cell 

 layer proceeding from a linear source of proliferation, the 

 gastrula ridge. It should be noted that the posterior and 

 superficial portion of this proliferating sheet is in all my pre- 

 parations marked by a somewhat different histological aspect, 

 distinguishing the component cells from the subjacent strata 

 of mesoblast. For this I refer to the figs, 56 — 60, and 83, 87, 

 91, but do not wish for the present to enter upon speculations 

 concerning the eventual significance of this histological dis- 

 tinction (cf. p. 549). 



That the peristomal mesoblast can indeed be compared to a 

 fan that is opened to its maximum width, can be noticed in 

 figs. 33—35 of stage 73 a, b, and d. The dotted line marks 

 off the outer edge of mesoblast ; it is more or less circular, 

 with the gastrula ridge for a radius.- 



■ In similar sense Kolliker expresses himself for the rabbit when he says 

 (' Pestschrift Wiirzburg,' Bd. i, 1882, p. 34) : " Angesicht gewisser neuerer 

 Erfahrungen iiber die Entstehung des Mesoderms aus paarigcn Aulagen 

 betone ich dass beim Kaninchen Axeoplatte uud Mesoderm bei ihrem ersteu 

 Auftreten eine zusammeuhangende Lage darstellen und dass auch das 

 Mesoderm bei seinem Weiterwuchcreu wenigsteus nach der einen hiuteren 

 Seite eine unpaare Bildung darstelt." 



- It must be observed that in these figures no dotted line is used to indicate 

 the boundary of the less sharply defined mesoblast that originates from the 

 protochordal plate, whereas the dotted line of figs. 62 — 64 and 79 — 81 does 

 include such mesoblast, which has then become fused both with the gastrula- 

 ridge mesoblast of figs. 33 — 35 and with the peripheral mesoblast that 

 originates out of the annular hypoblastic zone. 



