MUSEUM OF COMPARATIVE ZOOLOGY. 189 



with the amphiaster of the first cleavage nucleus ; for the latter has a 

 conspicuously eccentric position, — it lies nearer the animal than the 

 vegetative pole. 



2. The somewhat sharper limitations of the protoplasmic spheres 

 (stellate figures) than prevails at the time of the first and subsequent 

 segmentations. And, finally, — 



3.' The appearance presented by the thickened envelope of the vitellus, 

 which seems to possess less consistency than during the later stages, and 

 even at intervals to be altogether interrupted. This I take to be an 

 indication of want of age. 



I have not been fortunate enough to secure eggs in which the matura- 

 tion spindle exhibits a position oblique to the radius passing through its 

 centre, if I except a single specimen (Fig. 53), which I am inclined to 

 consider as a case presenting the second rather than the first archi- 

 amphiaster.=* ( See pp. 206, 207.) 



The next stage which has been observed (Fig. 45) is one in which the 

 first maturation spindle has a radial position, one of the stars being at or 

 near the centre, and the other reaching with its rays very near to the 

 surface of the vitellus. The peripheral extremity of the radius in which 

 this first spindle lies marks an important place in the topography of the 

 e^g. From this time on it forms a cardinal point, to which one may 

 refer all changes of form. It is the point which we have already desig- 



* The reasons for considering Fig. 53 as that of the second archiamphiaster, 

 instead of the first, are : — 



1. The egg is one of a series of seven taken immediately after their deposition by 

 the slug. Four of these were at once subjected to the acid, and all show the first 

 polar globule in an advanced state of formation. This was submitted to acid nearly 

 thirty minutes later, and consequently would have had time to accomplish the elim- 

 ination of the first polar globule, and effect the metamorphosis which its supposed 

 condition implies, provided it was equally advanced with the others. 



2. The appearance of the yolk at the surface near the more superficial of the two 

 asters. In the shallow depression of its surface and the ragged edges which it presents, 

 the yolk so completely resembles the appearance of the vitellus after the production 

 of a polar globule, that I am inclined to believe that the globule has been detached and 

 escaped notice. 



3. The smaller size of the peripheral aster. It is probable that the internal star 

 of the second archiamphiaster is the more direct (if not the exclusive) genetic suc- 

 cessor of the single star remaining at the closing phases of the detachment of the first 

 polar globule, and therefore may be expected to be soQiewhat more conspicuous than 

 its more recently formed companion. 



This last argument would have no weight if the second amphiaster arises, like the 

 first, from the metamorphosis of a typical nucleus, since then both stars would be of 

 equal age. (Comp. p. 206.) 



