MESODERM AND THE TAIL IN PETROMVZON. 



51 



In short, the outer and inner layers roofing over the archenteron in front 

 of the dorsal blastoporic lip are added to by the posterior grozvth of the 

 dorsal and lateral lips, by which the blastopore is reduced. 



During the reduction of the blastopore, in the inner layer which 

 passes round the blastoporic lips into the outer layer, can therefore always 

 be distinguished three divisions : the median stem in front of the dorsal 

 blastoporic lip and two limbs behind it, which latter constitute Avhat is 

 known as the peristomal mesoderm. 



This process of reduction must not be mistaken for the so called 

 " concrescence." The median stem is not brought about, as one might 

 assume, by coalescence of the two posterior limbs, but both the divisions 

 are the product of one and the same process : the blastoporic lips grow 

 along their whole extent in reducing the blastopore 1 '. The limbs become 

 apparent, because the growth commences its work at the mid-dorsal point 

 of the lip and proceeds both sides towards the lateral lips, so as to produce 

 in the latter part the posterior limbed division, and because in the sub- 

 sequent growth is kept the similar difference of progress concerning the 

 dorsal and lateral sections of the lip. The occurrence of the peristomal 

 mesoderm is, therefore, due to nothing but the delay which is displayed by 

 the lateral and ventral sections of the blastoporic lip in their growing. 



The last remnant of the blastopore, which alone is regarded by all 

 other observers as the blastopore, is raised up all around the ridge. Only 

 the dorsal lip is indented to a rounded notch which is dorsally shallowed 

 out into the dorsal groove. 



The overgrowth of the blastoporic lip, by which the blastopore is 



.1) This fact is in parallel with the discovery by MacBride (07) in Amphioxus. The author 

 pointed out the nuclear structure which is quite different according as the ecto- and entoderm 

 and therefore, by which the two germinal layers can fairly be put from each other. 

 I was fortunate enough by his kindness to have looked through the series of sections through 

 Amphioxus embryos in his possession and to have confirmed his account. This discovery 

 affords the direct and so strong positive evidence for the reduction of the blastopore by the 

 growth of its lips, that there is hardly room for assumption of the concrescence. In spite of 

 my efforts, I could detect in Petromyzon neither the parallel fact nor any other cytological 

 difference between the roof of the archenteron and the ectoderm over it, although Lwoff (94) 

 ■ distinguish them by the iize of yolk-granules, etc. from each other. 



