1890.] Conerescence Theory of the Vertebrate Embryo. 621 
Koller, 37; Disse, 75, z6; Waldeyer, 64, 65; M. Braun, 72; 
Gasser, 27, 22; Rauber, 26 ; C. Rabl, gz. 
The following descrip- 
tion applies to the hen’s 
egg. Whenthe egg is laid 
the centre of the segmen- 
ted blasto-disc presents a 
circular area of lighter 
color; during the first 
few hours of incubation 
this area pellucida, as it 
is called, becomes more 
distinct; as concrescence 
goes on the area pellu- 
cida expands, and the 
primitive streak appears 
in it excentrically be- 
tween : the eighth and 12.—AÀrea pellucida Ss a hen's egg with 
; compiete marc ive furrow; after Duval. do, area 
twelfth hour. By the opaca , anterior cre escent P area pellucida; 
sixteenth hour the primi- 7^ primitive gita 
tive streak has its full length. The rate of development is ex- 
_ tremely variable, autumn eggs developing more slowly than 
- spring eggs; the eggs vary also individually, and are moreover 
much influenced by the temperature of their incubation. For 
a fuller discussion of these variations see His, 25, 56-63. Seen 
from the surface the area pellucida with completed streak presents 
the following features: the area pellucida, ap, is considerably 
elongated and somewhat pear-shaped, being widest at the anterior 
end of the primitive groove, 77; this groove is well marked as 
a narrow and shallow furrow, which begins some distance from 
the anterior edge of the area, and ends just before reaching the 
posterior edge of the area; the front end of the furrow usually 
bends slightly to the left, but not invariably, as Koller and Rabl - 
have maintained, for it sometimes bends to the right or is quite 
straight; a line of granules is sometimes noticeable above the 
primitive groove; they were seen by Sharon 4. c., and are called 
by Duval, 77, p. 15, the flament épiaxial, e Gasser, 74. The 
