1886. } Eminyology. 183 
tion of the diameter extending from the centre of the blastodisk 
to the opposite pole. By this time the blastula has become 
adherent to the uterine epithelium through the intermediation of 
the transitory outer layer of cells, of (Reichert’sche Deckschicht), 
already mentioned, but the constituent cells of a certain portion of 
this outer layer, just overlying the germinal disk, as indicafed at s, 
rapidly proliferate, so as to form a lenticular or columnar thick- 
ening or mass, constituting what Selenka calls the Zréger, a term 
which may be anglicized by the word suspensor. This suspensor 
immediately overlies and pushes the germinal area or mass in- 
wards before it, down into the hollow cavity of the blastula. The 
germinal area is either pressed inwards into the hollow blastula, 
so that it assumes a concave form above, with a cavity between it 
and the lower surface of the suspensor, as in Arvicola, or the 
epiblast forms a solid mass, before which the hypoblast is pushed 
inwards by the ingrowth of the suspensor, so that the blastula 
assumes the form of an elongated sack, as in the ovum of the rat 
or the guinea-pig. 
€ process just described is somewhat similar to that of gas- 
trulation, for the germinal pole of the blastula is pushed down- 
ward into the sack formed by the hypoblast and outer layer, so 
that the embryo is finally developed quite at the opposite pole of 
the elongated blastula, as in the guinea-pig. The steps by which 
the mode of development of the embryo of the latter came to be 
established will be much better understood by reference to dia- 
grams C, D, E, and F, representing four stages of the develop- 
ment of the rat copied from Selenka! In these figures it will be 
obvious to the reader that the principal result of the precocious 
invagination of the embryonic area is to throw the embryo to the 
Opposite pole of the egg, and to so encroach upon the cavity of 
the mesenteron, the umbilical vesicle, as to almost obliterate it, as 
is shown in Fig. The embryo Æ is also bent into a curve, just 
the reverse of that shown in Fig. 8. The ccelomic space ¢ is also 
more restricted, and the sinus terminalis s¢,in Fig. F, seems to 
—S über Entwickelungsfeschichte der Thiere. Drittes Heft. Die Blätter- 
im Ei der Nagethiere, 4to. Wiesbaden, Kreidel, 1884. 
