48 
as the plates of a calcareous shell, and compares the egg to> 
a sea-urchin. 
To return to the segmentation ; immediately after the large 
vesicles have disappeared, the egg has the appearance shown 
in Figure 14. One minute later (Figure 15), the first mi- 
cromere, c, has become nearly spherical, and stands out sharp- 
ly from the remainder of the egg, and the compound mass, a, 
and b, of the preceding figures, is again separated into the 
macromere, a, and the second micromere, b. In another min- 
ute (Figure 16), the formative pole is divided into four mi- 
cromeres, one of which, b , is only slightly marked off from 
the macromere, while the three others are more distinct. 
The impossibility of seeing both sides of the egg at once 
makes it difficult to say just how these four spherules are 
formed, but it seems most probable that two of them are 
formed by the division of the first micromere, and two by 
the division of the second. 
In the egg which was figured the nearest spherule bears 
every indication of an origin by the division of the first mi- 
cromere, c, into two, and other eggs served to show with equal 
clearness that one of them is also separated off from the 
second micromere, b. Certain irregular forms of segmentar 
tion, which will be described later, also appear to sustain this 
view. 
Owing to an unfortunate oversight, the dotted lines which 
should connect the letters of reference with the parts they 
refer to were not copied in the drawings from which the photo- 
electrotypes of Plates II. and III. were made; but I hope 
that a more careful description will supply a remedy for the 
accident. Two minutes later, Figure 17, three of the micro- 
meres, the first, c, and the two new ones, d d , are well de- 
fined and prominent ; but one of them, the second micromere,. 
b , has again begun to become fused with the macromere, a. 
After another interval of three minutes and a half, Figure 18,.. 
this micromere has become completely fused with the macro- 
mere, to form a compound mass, a and b, which is almost 
spherical, and in this a single transparent vesicle has made its. 
appearance. The other three Jmicromeres are even more 
