STUDIES IN MAMMALIAN EMBRYOLOGY. 511 



embryo. Still it was soon found that this disadvantage was 

 imaginary; that, the series being complete, entire accuracy of 

 reconstruction could be obtained, and that^ more careful perusal 

 and comparison being necessary under these circumstances, 

 certain points came to light which otherwise might have re- 

 mained unnoticed, whereas certain other points are much less 

 evident in either transverse or longitudinal section series than 

 in oblique ones. 



The utmost care was bestowed on the surface views, which 

 have been traced by means of direct reconstruction (in a given 

 plane of projection) of the camera lucida drawings that were 

 made of the actual sections. And so we commence our descrip- 

 tion with what occurs in the anterior source of mesoblast — the 

 protochordal plate. 



This plate of thickened hypoblast in embryo 73/ (fig. 32) 

 can hardly be said already to contribute to the formation of 

 independent cells between hypoblast and epiblast (figs. 40 and 

 41). But in 73b (fig. 33) it does thus contribute. In the 

 centre its original character as a thickened patch of hypoblast 

 is retained, and even more marked by still more considerable 

 local proliferation (figs. 53 and 54). At the borders of this 

 patch fusiform cells are seen (in sections not here figured) to 

 radiate from it, and to spread between the two primary layers 

 in a direction more or less perpendicular to the long axis of 

 the embryonic shield. 



Thus the first trace of lateral wings of cells (that have 

 originally a decidedly fusiform '^mesenchymatic '^ aspect) de- 

 veloping from the protochordal plate becomes marked. 



In embryo 73d (fig. 34) the mesoblast-cells in this anterior 

 region have already attained a more considerable numerical 

 development (figs. 43 and 44), it being at the same time very 

 worthy of note that they do not to any extent spread back- 

 wards, i. c. in the direction of the front end of the gastrula 

 ridge. 



In the following stage, represented by the embryos No. 45 

 (figs. 62 — 64), the mesoblast is already one continuous plate. 

 But in the anterior region of the embryonic shield we easily 



