DETAILS OF EARLY EMBRYO. 83 



faded away, and, part passu, the yolk nuclei have greatly increased in number. In 

 the region where the yolk-entoderm approaches the lumen of the gut it thickens 

 and sinks downward, leaving as the floor of the gut cavity a wedge-shaped mass of ger- 

 minal yolk. At the outer rim of the yolk-entoderm we observe that it becomes con- 

 tinuous with the mesoblast; in other words, recalling sections D and G, the peristomial 

 mesoblast of Chimaera which now arises is not continuous with the gastral mesoblast. 

 We have thus a reason for inquiring whether gastral and peristomial mesoblast 



Figs. 71 J-N. Details in sections of foregoing embryo (figs. 71 A-I). 

 /. Region of peristomial mesoblast. 



e, ectoderm; e', cells recently derived from e ; ent t entodcrm ; m, perutomiat mesobiast. 

 K. Detail of subgerminal yolk region showing cellular arrangement of merocyte elements. 

 L. Lying in the subgerminal yolk is a megasphere, which, on the evidence of the overlying vacuoles, is in the process of rising 



towards the yolk entoderm. 



M. Similar megasphere passing into the yolk entoderm. 

 N. Megaspheres similar to preceding, but representing a somewhat later stage of passage into the blastoderm. 



are as intimately related as we have generally assumed.* A condition of the 

 peristomial mesoblast is figured in detail in j, and it proves of considerable interest, 

 since the region of mesoblast proliferation is of wide extent. Not only are cells 

 budded out from the marginal mass m, but we observe also that cells are added to 

 the mesoblast from the neighboring ectoderm ; thus at e' is a cell which has been 

 derived from the ectoderm e, where, by the way, a syncytium is now present ; and 



*Cf. the current view as to the secondary confluence of blastopore and yolk " blastopore, " as summarized in 

 Ziegler's Handbuch der Embryologie, pp. 352 and 353. 



