510 A. A. W. HUBRECHT. 



limits of the embryonic shield, and thus enclosing — but at the 

 outset independent from — the protochordal plate. 



This annular zone makes its first appearance when the gas- 

 trula ridge has already passed through its first stages of 

 development. Its formation is thus a later phenomenon of 

 difi'erentiation in the hypoblast than was that of the proto- 

 chordal plate. 



Still it is of the same order, being essentially in the first 

 instance a thickening and approximation of a number of hypo- 

 blast-cells that are contained in the annular zone below, but 

 just outside the border of the epiblastic shield. This change in 

 the hypoblast-cells appears to take place almost simultaneously. 



Embryo 73/ (fig. 32) was taken from the same uterus and 

 preserved in the same way as the embryos 73a, b, and d (figs. 

 33 — 35). In it there was not yet a trace of the zone here 

 alluded to, and also in other respects this embryo is somewhat 

 behind the others in development. In the three others the 

 closed ring can be discerned which is formed of the hypo- 

 blast-cells here alluded to ; the formation of the ring may 

 hence be concluded to take place rather suddenly. 



There will be some difficulty in giving a concise description 

 of the origin of the mesoblast from the three diff'erent sources 

 here stated, because of the very early tendency of their pro- 

 ducts to become mixed up into one continuous layer. Only at 

 the very earliest stages is the distinction a clear one. And 

 so it is more especially these which will have to be described 

 somewhat more fully. The embryos No. 73, above alluded to, 

 are for this purpose all the more valuable, as they differ in 

 development just enough to elucidate the actual course of the 

 phenomena we are here investigating. 



It must here be remarked that there is an apparent disad- 

 vantage in the fact that some of the sections are neither strictly 

 transverse nor strictly longitudinal. This could not always be 

 attained, since it had become obvious that the best way of 

 cutting the blastocyst was to imbed it in situ in the uterus, 

 even though in the early stages there is no strict con- 

 cordance between the long axis of the uterus and that of the 



