558 A. A. W. HUBKECHT. 



PLATE XXXVIII. 



Figs. 32 — 35. — The embryonic shield of four blastocysts that were taken 

 from the same mother (Cat. n°- 73). These tracings were obtained by the 

 reconstruction of the details of structure in the different regions of the 

 embryonic shield from the unbroken series of sections. The epiblastic shield is 

 marked by a dark outline. The annular ring of modified hypoblast has a 

 uniform grey tint, and so has the protochordal plate, which was also already 

 indicated on the foregoing plate. 



The gastrula-ridge makes its first appearance in Eig. 32 ; in the other 

 figures it is much farther advanced, and traces of a gastrula-groove are found. 

 The regions right and left of the gastrula-ridge, as well as behind it, contain 

 the continuous plate of mesoblast, of which the outer boundary is indicated 

 by a dotted line. The arrows have reference to the different sections as 

 figured on this and the next plates ; the numbers attached to them are the 

 same as the respective numbers of the figures in which those sections are 

 represented. If the sections do not extend through the whole length of the 

 embryonic region, the arrows are correspondingly shortened. 



In Fig. 34 the front part of the annular ring of modified hypoblast is 

 omitted, because this portion being bent in the plane of the section 

 could not allow its outlines to be rigorously traced in the reconstruc- 

 tion process. 

 In Eig. 32, although obtained from the same uterus, no annular zone of 

 modified hypoblast was as yet present. This zone may consequeutly 

 be said to appear as a simultaneous modification of the region in 

 question. 

 Figs. 36 — 39. — Four sections through the posterior region and first origin 

 of the gastrula-ridge of the embryonic shield of Fig. 32. In four sections, 

 two of which are figured, there is adhesion between the proliferating epiblast 

 (palingenetic hypoblast) and the subjacent layer of flat cells (ccenogenetic hypo- 

 blast). At the front end of the incipient gastrula-ridge there are faint traces 

 of a perforation indicated by p. in Figs. 38 and 39. pto. Cell-mass in front of 

 this : incipient protochordal wedge (cf. Figs. 46 — 48, and 55, pw., of slightly 

 older stages), ffr. Proliferating cell-mass behind it : incipient gastrula-ridge. 

 Fig. 36.— Mas. Utr. Cat. n"- Sorex 73/, 2 r. 15 s. 

 Fig. 37.— „ „ „ 17 s. 



Fig. 38.— „ „ „ 19 s. 



Fig. 39.— „ „ „ 20 s. 



Figs. 40 and 41. — Two sections through the anterior region of the embry- 

 onic shield of Fig. 32. pp. The hypoblastic protochordal plate, already more 

 than one cell thick. 



Fig. 40.— Mus. Utr. Cat. u°- Sorex 73/, 2 r. 19 s. 

 Fig. 41.- „ „ „ „ 21*. 



