MUSEUM OF COMPARATIVE ZOOLOGY. 2G9 



larva from the side opposite that on which the primitive hydrophyUium 

 is attached. The primitive cavity is thus thrown behind the yolk, and 

 is concealed by the buds which have already appeared, one of which is 

 shown in profile. The larva is placed in what is considered its normal 

 position comparable with the natural position of the adult. The float 

 is well developed, and resembles closely that of the adult. Below it 

 there is a well-marked red pigment-spot on the external walls of the 

 ovum, which forms a convenient point for the orientation of other organs, 

 and which itself forms in later stages a well-known organ (embryonic 

 tentacle) ; and at the pole of the egg opposite the float we find the 

 partly formed polypite. The lower part of the large transparent body 

 behind the yolk is the distal rim of the hydrophyUium ; the upper part is 

 the proximal border. The axis of the future Agalma is thought to pass 

 lengthwise through the float, and to cut also that pole of the yolk at 

 which the polypite is forming. 



The axis of the larva, as thus indicated, does not coincide with that 

 which originally passes through the egg from the point at which the first 

 elevation of epiblastic and hypoblastic layers took place to the opposite 

 pole. It is apparently at right angles to this. If I am right in regard 

 to the relationship, or, to use a stronger word still, the coincidence, of the 

 former axis with the first plane of cleavage in the unsegmented ovum, 

 the axis of the adult Agalma is at right angles to the first plane of 

 cleavage. It may be mentioned at this point, that in the gonophore, 

 as the egg first forms, the axis of the ovum passing through the red 

 pole and the point of attachment of the gonophore is normally at right 

 angles to the axis of the Agalma. The horizontal diameter of the larva 

 at this stage is .70 mm. The vertical diameter is .75 mm. The longer 

 axis of the ovum is .45 mm.; the shorter, .35 mm. 



Fig. 1, PI. IV. is taken from a larva a little older than the last, but 

 still five days old. It resembles the young Agalma Sarsii at the close 

 of the second week. The axis is placed vertical in the same position as 

 that of the adult as usually represented. The separation of the hypo- 

 blast from the yolk-cells has left a cavity of relatively considerable size 

 at the point where the polypite has beguu to form. This cavity recalls 

 a similar cavity in the larva of Crystallodes as figured by Heeckel. 

 There is as yet no apparent diminution in the size of the primitive 

 hydrophyUium, and the outlines of the epiblastic cells upon it can be 

 easily traced. The yolk-cells still enclose the protoplasmic network, 

 and have the same polygonal shape as earlier in their history. The 

 float is more elongated and lies on one side of the yolk. It is filled 



