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 tenxis 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 



VOL. XI. NO. 11. 17 



