328 PROFESSOR J. COSSAR EWART. 



Plate XIII. 



Fig. 28. Section of the blastocyst immediately beyond the sinus terminalis. tr., trophoblast consisting 

 of columnar cells — note that the cells are in contact with each other along their whole length, have square-cut 

 ends and deeply placed nuclei ; md., unsplit mesoderm lying in a space between the trophoblast and the 

 yolk-sac endoderm ; md.e., growing edge of mesoderm, in contact with inner surface of trophoblast ; y.e., 

 yolk-sac endoderm, in contact with the trophoblast, immediately beyond the growing edge of the mesoderm; 

 !>.>:., a blood-vessel beyond the sinus terminalis. 



Fig. 29. Section through the centre of a trophoblastic disc. The disc consists of very tall cells, devoid, 

 except round the margin, of sac-like processes. Some of the cells contain one or more round bodies derived 

 perhaps from the " uterine milk." The cavity of the disc is occupied by a deeply stained coagulum con- 

 tinuous with a membrane-like coagulum (cm.) lying between the trophoblast (tr.) and endoderm (end.). 

 The endoderm between the discs consists of a single layer of cells, but opposite the discs it sometimes consists 

 of masses of cells in contact with a lightly stained material which projects into the cavity of the yolk-sac. 

 Many of the cells surrounding the disc end in sac-like processes (pc). 



Fig. 29a. Section through the edge of a disc. In such sections, owing to the presence of many nuclei, 

 the disc seems to have resulted from a heaping up of cells. Many of the endoderm cells (end.) seen in this 

 section diner from the typical endoderm cells in figs. 30 and 31. 



Fig. 30. This section represents an early stage in the development of a disc. On the left of the section 

 are nearly typical columnar cells (tr.) ; then come cells with sacdike processes (pc.), apparently in the act of 

 taking in food particles ; then cells which by proliferating and elongating eventually form the disc ; cells with 

 sac-like processes (pc.) also occur on the right of the elongating cells. In the region of the developing disc 

 the coagulum is thick and irregular ; beyond this region it has the appearance of a basement membrane. 

 Further, within the developing disc the endoderm has undergone proliferation and lies in contact with a 

 lightly stained coagulum. 



Plate XIV. 



Fig. 31. A minute coagulum lying between the trophoblast and the endoderm. This represents the first 

 stage in the development of a trojmoblastic disc and of a yolk-sac tubercle ; tr., trophoblast ; end., endo- 

 derm ; cm., coagulum. x 400. 



Fig. 31a. The coagulum in fig. 32 seen from the cavity of the yolk-sac. The cells in contact with the 

 coagulum are more granular and more deeply stained than the surrounding endoderm cells. x 400. 



Fig. 32. A later phase in the development of a yolk-sac tubercle. The coagulum (cm.), now larger, is 

 invested by highly granular endoderm cells (end.) x 400. 



Fig. 32a. Section through same tubercle beyond the margin of the coagulum. x 400. 



Fig. 33. Diagram of longitudinal section of 21-days embryo and its membranes. Note the complete 

 amnion (am.), the small allantoic diverticulum (al.d.) embedded in the allantoic mesoderm (al.m.), the wide 

 yolk-stalk (ys.), that the mesoderm (md.a.) except in the immediate vicinity of the embryo is still unsplit, 

 and that the spinal cord opens at its caudal end (c.n.) into the amniotic cavity (am.c.) ; at , atrium; cl., 

 cloaca; d.a., dorsal aorta; ecd., ectoderm; end., endoderm.; exo., exocoelom ; f.b., fore-brain; h.b., hind- 

 brain; m., "longitudinal growth-centre of embryo"; m.b., mid-brain; md., mesoderm; mo., mouth; 

 nc, notochord ; nex., canal of spinal cord; p.o.v., optic vesicle; s.c, spinal cord; s.v., sinus venosus ; 

 t., tuberculum impar; v., ventricle; y.sa., yolk-sac; 1, 2, 3, 4, internal branchial slits. Circa 15 times 

 natural size, 21-days embryo. 



Fig. 34. Semi-diagrammatic drawing of the 21-days blastocyst with part removed to show the embryo. 

 Note the complete amnion (am.), the yolk-stalk (y.s.) leading into a large yolk-sac, the left vitelline artery 

 (v. a.), the sinus terminalis (s.t.), and the right vitelline vein (v. v.), that the greater part of the mesoderm is 

 still unsplit (md.a.), and that the non-vascularised distal portion of the blastocyst consists only of tropho- 

 blast (tr.) and endoderm (end.). Beyond the sinus, dots (t.d.) represent some of the trophoblastic discs. 

 am.c, amniotic cavity ; exo., exocoelom ; md., mesoderm. Circa 3 times natural size. 



Fig. 35. The 21-days embryo as it appeared when the chorion was removed. Note the large left 

 vitelline artery (v.a.), the large right and left vitelline veins (v.v.), and the rich plexus of vessels in the 

 vicinity of the embryo. 



