MUSEUM OF COMPARATIVE ZOOLOGY. 205 



It is an important question, How does the second archiamphiaster 

 arise, and what relation does it bear to the first archiamphiaster 1 



Very few observers have given this question special attention, and 

 those who have are not all positive in their opinions. According to most 

 of the descriptions given, the vitelline " half-spindle," which remains 

 after the formation of the first polar globule, simply undergoes an 

 elongation caused by the gradual recession of the single remaining aster 

 from the surface ; the internal zone of fibre thickenings disappears by 

 the distribution of its substance to form the lengthening fibres of the new 

 spindle ; and there arises a second stellate figure whose rays converge 

 toward a point of the surface where the peripheral end of the spindle 

 remains. Such an origin could not be directly compared with the for- 

 mation of the first archiamphiaster, or subsequent amphiasters : it must 

 be at best a greatly abbreviated process, if at all comparable with the 

 ordinary method of amphiastral formation. In all other cases both cen- 

 tres of radiation arise as new differentiations in the protoplasm, and only 

 make their appearance when the nuclear substance has assumed a defi- 

 nitely circumscribed form ; in this case (according to the authors) only 

 one of the centres of radiation has the least claim to be considered new, 

 and the nuclear thickenings do not become fused into a definitely limited 

 nucleus. 



The case (Fig. 23) to which I have called attention presents some 

 evidence that the second archiamphiaster is not formed in so direct 

 a manner as has been supposed. There is no absolutely incontroverti- 

 ble reason for denying that this complete amphiaster may have been 

 formed much in the manner above indicated for the incomplete one. 

 It would only be necessary to assume an extensive migration from the 

 surface on the part of the spindle and its asters, instead of a movement 

 on the part of the deep aster alone. There are, however, some objec- 

 tions to this view. The spindle has the appearance of being formed 

 in the ordinary way, rather than that of having its fibres drawn out ; it 

 is not so sharply defined as I should expect a spindle to be, if resulting 

 from a drawing-out process ; it is much broader, and its peripheral fibres 

 more abruptly bent, than would be the case in that event. The fact of 

 its being totally enveloped in the yolk is in itself more easily reconcilable 

 with its formation in a normal than in an abbreviated manner, since in 

 the former case the centres of radiation arise at points within the vitel- 

 lus, and thus is avoided the necessity of supposing that there is a cen- 

 tripetal migration of the spindle. 



We have seen that at the completion of the first polar globule the 



