534 A. A. W. HUBRECHT. 



Triton and Petromyzon — (1) to a further excavation of an 

 archenteric cavity in the direction which in this figure is 

 marked by five blue dots ; and (2) to the simultaneous growth 

 of notochord and paired mesoblast plates (gastrales raesoblast, 

 Rabl) in that advancing anterior region. 



It was above noticed that authors do not agree as to the 

 extent of mesoblastic cell material yet produced by delamina- 

 tion from the hemispheric lower surface of the hypoblastic 

 cell-mass, nor does it matter for the argumentation here put 

 forward. Still I notice this point because by-and-bye we will 

 have to reconsider this possibility, and will then have to 

 picture to ourselves the hemispheric surface here alluded to, 

 which in this diagram has received a uniform blue tint, and 

 forms the lower layer of the hypoblastic tissue. 



Recapitulating, we thus find that in this figure the round 

 dots, both the white and the blue, are meant to designate the 

 zone where notochord and gastral mesoblast originate; the white 

 stripes, the zone where the peristomal mesoblast arises ; and 

 part of the uniform blue hemispherical region, the zone of what 

 I will designate as the peripheral mesoblast. 



The passage from this stage to one in which the yolk has 

 very considerably increased, as is the case in Sauropsida 

 embryos, has already been described by Rabl ; his figure of a 

 diagrammatic longitudinal section is reproduced with a very 

 slight modification and without colours in fig. 94. The blas- 

 topore, the anterior lip of which is at the same time the 

 anterior surface of the neurenteric duct, is easily identified in 

 both figures. In front of this anterior lip (i. e. to the left in 

 RabFs figure) is the embryonic region ; to the right is the 

 region of the primitive streak, where the lips of the blastopore 

 may be said to coalesce. The regions indicated by white dots 

 and by white stripes in fig. 93 (notochord and peristomal 

 mesoblast) are here indicated by the letters pw. and pst. ; 

 below them is a sheet of cells, the hypoblast, which is always 

 distinct, although its significance has lately been diflferently 

 interpreted (paraderm, Kupff'er; lecithophore, van Beneden), 

 and which I have here somewhat more distinctlv indicated 



