1890.] Concrescence Theory of the Vertebrate Embryo. 705 
The appearances in a sheep’s ovum at this stage are illustrated by 
Fig. 18 of a vesicle of 12-13 days from a sheep; the vesicle meas- 
ured 55 mm. in length by about 1.5 in breadth, but the length of 
the vesicle is extremely variable at this stage; the specimen had 
been stained to bring out the small close-set nuclei of the outer 
layer, and the larger, more widely-set nuclei of the inner layer. 
The upwards-arching embryonic shield, SZ, shows Hensen’s knot, 
kn, and the beginning primitive streak, around the edge of the 
shield the middle layer makes an irregular shadow, mes. 
A condition of the blastodermic vesicle similar to that described 
is figured by Coste for the rabbit, by Bischoff for the rabbit 6, Taf. 
IX, Fig. 42, C,—for the dog, 
7, Taf. nr, Fig. 28, B; and the 
gradual extension of the second 
layer is recorded for the mole by 
Heape, 23. Since it is known to 
occur in rodents, carnivora, and 
. insectivora, it is probably true of 
all placental mammals that the 
one-layered vesicle becomes two- 
layered by the outgrowth of cells 
for the *inner mass" found at 
the close of segmentation; this 
is the first step of development 
after segmentation. 
Rauber's Deckschicht has evi- 
: dently great importance. It was 
ceca ie portion of a sheep’s first described by him in the 
O 
ic vesicle of 12-13 days. Sh, : ? : 
M. Messer knot, wey, * Mews rabbit, 25 ; and was also dis- 
Shield; 4”, H 
blasthof” ; after Bonnet, 34 diams. covered by E. Van Beneden, 7, 
who, however, made the blunder of considering it as the perma- 
nent ectoderm, and the true ectoderm below it as the mesoderm ; 
this error has been amply corrected by Kölliker, and is now 
admitted by Van Beneden (see Van Beneden and Julin, 7). Its 
disappearance in the rabbit has also been studied by Lieberkühn, 
41. Balfour (Comp. Embryol, II., 219), from investigations on the 
rabbit by himself and Heape, concluded that the cells of the deck- 
