STUDIES IN MAMMALIAN EMBETOLOGT. 533 



At all events, these various considerations allow us to catch a 

 glimpse — however hypothetical — of the causes that may have 

 brought about the necessity of the precocious segregation of part 

 of the hypoblast in mammals. And we can very well understand 

 that once the double closed sac having been constituted, the 

 further processes should again offer close analogies to what is 

 observed in Sauropsida. The precocious segregation has not 

 necessarily had any altering influence on the hereditary 

 tendencies of the different portions of the hypoblast from 

 which mesoblast originates. As might be expected, the palin- 

 genetic phenomena are more closely comparable ; the cenoge- 

 netic changes do not, however, in any way escape the possi- 

 bility of comparative analysis. 



I will now attempt to further elucidate the argumentation 

 contained in the foregoing pages by the discussion of four 

 diagrams given on PI. XLII. 



Fig. 92 is a diagram of a developmental stage in either 

 Cyclostomata or Amphibia after the infolding has commenced, 

 and when from the medio-dorsal wall of the archenteron both 

 the notochord and the lateral plates of gastral mesoblast have 

 developed, whereas at the lower lip of the blastopore peri- 

 stomal mesoblast is originating. If for a moment we give no 

 attention to the different colours in this diagram, nor to the 

 fact that the solid mass of hypoblastic yolk-cells is here only 

 represented by a few polygonal outlines, we know that the 

 continuation of the process just commenced leads both in 



that the incriminated spot is found. Judgiug from Selenka's figures, it would 

 seem to be the starting-point from whence the inwandering of cenogenetic 

 hypoblast commences, and he on purpose applies the name blastopore both to 

 it and to a yet earlier stage in which he noticed an actual displacement 

 inwards (1. c, pi. xvii, fig. 8). It is all the more suggestive that this spot is at 

 all events in the immediate vicinity of what will become the first trace of the 

 gastrula ridge, i. e. of the point of origin of the palingeuetic hypoblast 

 appearing so much later, when the cenogenetic is already a closed sac. 



That in Sorex the case lies somewhat differently, and that here no corre- 

 sponding thinner spot is noticed, must no doubt be ascribed to the thickness 

 of the epiblastic embryonic shield, which is comparatively considerable even in 

 the very earliest stages. 



