1890.] Conerescence Theory of the Vertebrate Embryo. 625 
bation,—show three changes clearly. A cross section through 
the area opaca in front of the area pellucida shows the thin ecto- 
derm, the thick cellular entoderm overlying the archenteric 
cavity and charged with yolk granules ; the entodermic nuclei are 
very variable in form and irregular in distribution; the cell boun- 
daries are indistinct. There is no mesoderm. A cross section 
near the front of the area pellucida likewise shows only ectoderm 
and entoderm ; the former is a high cylinder epithelium over the 
area pellucida and thins out towards the opaca on each side; the 
latter is a thin layer over the area pellucida, and passes quickly 
but not abruptly into the very thick yolk-bearing entoderm (or 
Keimwall) of the area opaca. Sections a short distance in front 
of the primitive groove show that the head process (Aopfortsatz) 
is a forward prolongation of the primitive streak and consists of an 
FIG. 15.— Transverse section of the anterior region of a fully develoyed primitive 
streak of a hen's ovum. Er, ectoderm; mes, mesoderm; Ent, entoderm; Pr. g, primi- 
tive groove. The black dots represent yolk granules. 
axial accumulation of the mesodermic cells, fused with the ento- 
derm, and having broad extensions sideways to form the mesoderm 
beeween the outer and inner germ-layers ; the lateral portions of 
the mesoderm have no connection with the outer germ-layers, and 
at its distal edge the mesoderm thins out and rests upon the ento- 
derm of the opaca, but without being connected with it; I cannot 
find any satisfactory evidence that it receives any additions from 
the opaca entoderm, as many authors have maintained. The ecto- 
derm in the region of the Kopffortsatz resembles that further 
forward, but it very soon shows a faint median furrow, the so- 
called dorsal groove (Rückenrinne), which is the commencement 
of the medullary groove. In the anterior half of the primitive 
streak the relations are similar to those in the head process, ex- 
