MUSEUM OF COMPARATIVE ZOOLOGY. 257 
in the formation of that polar elevation which marks the origin of the 
primitive hydrophyllium, the first-formed organ of the larva. The va- 
rious designations which have been used in the nomenclature of the two 
poles of the egg in this and following stages admit of misinterpretations. 
If we call the pole at which the increase of the thickness in the surface 
layer takes place the upper pole, we convey a wrong impression as to its 
natural position in the water; for if we observe the position in which 
the egg floats in stages a little older, it will be seen that the so-called 
upper (“obere”) pole is always downward, as it naturally would be 
brought in equilibrium by the increase in weight resulting from the 
growing organ. Not less misleading are the terms oral and aboral. 
When the mouth of the first-formed polypite appears, it is in a position 
90° from that pole (the area germinativa) at which the primitive hydro- 
phyllium first forms. The aboral pole is therefore 90° from the position 
assigned to it, if the terms have anything more than an arbitrary sig- 
nificance. The rosy color seen at one pole of the unsegmented egg dates 
from the time.when the ovum was in the sac within the gonophore. At 
that early stage the pole of the ovum opposite the attachment of the 
sac is rosy in color, and through all stages of cleavage up to one with 
eight cells that same rosy pole has been recognized. Here (8-celled 
stage) the relations to the axis were lost; but a rosy region was still to 
be seen, and it seems legitimate to conclude that the rosy pole is identi- 
cal in these cases, rather than that the color has migrated from one 
region of the ovum to another in unseen stages intermediate between 
those submitted to exact observation. Moreover, going a step farther, 
can we not also regard that pole where the single layer is beginning to 
thicken, and which has the same reddish color, as identical with those 
which we have studied? I think we can suppose that the rosy color in 
this stage indicates the same pole which is marked out by it at the very 
beginning, —the same, in fact, through which the first cleavage plane was 
observed to pass. Although I have spoken of this pole as the germina- 
tive pole, its axis is not the same as the axis of the adult animal. The 
investing layer spread over the surface of the egg is thickest at the 
germinative pole, and diminishes in thickness gradually to the opposite 
pole. The thinning out of this layer is a regular diminution on all 
sides ; and up to the present time there are no right and left sides to 
the layers which cap the germinative pole. 
In the next stage (PI. III. fig. 3) following the last, the ovum, instead 
of being spherical, has become more elongated, assuming the form of a 
prolate sphere, and the portion directly under the germinative pole has 
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