UTERINE SPINDLE OF PLANOCERA INQUILINA. 28 1 



other forms, both among vertebrates and invertebrates. Conklin 

 ('02 and '12) has described this same phenomenon in Crepidula, 

 and in C. plana he states ('12) that the first maturation spindle 

 undergoes a reduction from about 42 n to 24 /i in length. He 

 ('02) cites the following references in which the shortening 

 process is known to occur: Ascaris (Boveri, '87), Branchipus 

 (Bauer, '92), Ophryotrocha (Korschelt, '95), Myzostomum 

 (Wheeler, '95), Cerebratulus (Coe, '99), Polychoerus (Gardiner, 

 '98), Axolotl and Triton (Carnoy and Lebrun, '99, as seen in 

 their figures no and 112). To this list we can now add Plano- 

 cera, and also Graffilla (Patterson, '12). 



Conklin's suggestion concerning the cause of this phenomenon 

 is of interest here. He believes that it is due primarily to the 

 peripheral movement of the spindle, and that its chief result 

 is the formation of a much smaller polar body than would be 

 produced if the spindle retained its original length. 



During the progress of the contraction, the spindle moves 

 toward the periphery of the egg, and upon reaching the surface 

 its distal end comes in contact with the egg membrane, which 

 apparently moves down to meet it. In the meantime the chro- 

 mosomes undergo division, and the two groups have reached a 

 late anaphase (Fig. 12). A protrusion on the surface of the egg 

 then appears and into this elevation the distal end of the spindle 

 is pushed (Fig. 13). Subsequently a typical polar body is cut off. 

 We have found one unusually clear case of the first polar body, 

 in which the ten chromosomes are easily seen (Fig. 14). Within 

 another hour the second polar body is thrown off in the usual 

 manner, and fertilization and cleavage then follow. In other 

 words, the whole process of maturation and fertilization in this 

 animal is quite typical of that of many other forms. 



In conclusion we may state briefly the results of our study. 

 We find that the so-called "uterine" or "aborting spindle" 

 of Planocera is initiated by the process of insemination; that 

 it is nothing more nor less than the first phases of a rather 

 typical maturation spindle, and that consequently it undergoes 

 a shortening while moving to the surface of the egg to give rise 

 to the first polar body. It may be going too far to suggest that 

 probably the uterine spindles which have been described in 



