32 Primitive Streak and NotocJiordal Canal in CJiclonia. 



According to Mitsukuri {^"^6, p. 28) the so called plug is composed 

 of undififerentiated cells. Behind the plug the ectoderm extends 

 over the whole surface of the streak and proliferates cells along 

 the axis into the cell mass below. Will, on the other hand, con- 

 siders the plug and streak to be composed of exposed entoderm. 

 In the gecko a primitive groove does not exist on the streak until 

 the open blastopore becomes elongated in an antero-posterior 

 direction. He says ('92*', p. 132), " Die beiden Schenkel der 

 winklig geknickten vordern Urmundlippe nahern sich einander 

 immer mehr und mehr, so dass sie einander bald parallel verlaufen 

 und dadurch eine Primitivrinne entsteht, deren Rander von der 

 Urmundlippe selbst, deren Boden von dem Entodermpfropf, resp. 

 dem Primitivstriefen gebildet wird " ; and on page 141 he says 

 that this union of ectoderm begins to take place first at the 

 anterior lip of the dorsal notochordal opening, and spreads gradu- 

 ally posteriad along the streak. Will ('93, Taf. 34, Figs. 17^-17^^ 

 and Taf. 35, Figs. iS'^-iS'') represents transverse sections through 

 the streak region of a Cistudo, in which the horns of the noto- 

 chordal opening have begun to turn posteriad. In these sections 

 a distinct separation is shown between the entodermic streak and 

 the ectoderm, and the ectodermal borders still lie at some distance 

 from the axial line of the streak. 



In the present instance (Plate I. Figs. 4 and 4') the horns of the 

 blastopore have only just begun to bend backward, and yet a deep 

 groove extends along the whole surface of the so called plug and 

 streak region. Cross sections show that a deep groove exists in 

 this region. After these drawings were made the embryo suffered 

 somewhat from overheating in preparation for sectioning, so that 

 I will not describe its histological condition. There is the less 

 need of it, since I possess two other embryos, whose histological 

 condition is faultless, which show the same groove. 



Figure 7 (Plate II.) represents an embryo possessing a much 

 shorter streak region, but a groove such as existed in the embryo 

 just described extends along its axis. This embryo (Fig. 7) was 

 cut parallel to the sagittal plane, and therefore does not furnish such 

 suitable sections for studying the groove question as are afforded 

 by series cut crosswise to the axis. Another embryo, which in 

 surface view so resembled Figure 7 that it is unnecessary to figure 

 or describe its surface appearance, was cut crosswise, thus furnish- 

 ing the necessary sections. Figure 50 (Plate X.) represents the 



I 



