214 bulletin: museum of compakative zoology. 



in Pjropbacus, is^ as yet, entirely unknown. This schizogony takes tlie 

 form of an obh'que fission of the entire organism whereby not only the 

 nucleus and cell plasma are divided but the parental skeleton as well. 



Division of Ancestral Skeleton. 



This skeleton is made up of a definite number of plates with a typical 

 arrangement throughout all the species of the genus. It is parted at 

 schizogony along existing sutures between the plates by the fission plane 

 which is oblique to the major axis, passing at an angle of approximately 

 45° from the right anterior shouhler of the midbody to the left posterior 

 region at the outer side of the base of the left antapical horn. By this 

 process the ancestral skeleton is parted (see plates 1 and 2) so that the 

 anterior schizont receives apical plates 1-4', precingulars 1" and 2", the 

 two girdle plates between the proximal end of the girdle and a point 

 near the middorsal line, and postcingular plates r"-3"', in all 9 plates 

 together with the pi'oximal half of the girdle and anterior moiety of the 

 so-called ventral plate. The posterior schizont receives precingulars 

 3"-4", the two girdle plates between its middorsal gap and its distal end, 

 postcingulars 4'" and 5'" and antapicals 1"" and 2"" ; in all 6 plates, the 

 distal half of the girdle and the posterior moiety of the ventral " plate." 

 The ventral plate, as Lauterborn (1895) has shown, is divided between 

 the two schizonts by a more or less oblique line, which passes from the 

 flagellar pore at the proximal end of the girdle to the attachment area at 

 its distal end. The position of the fission plane with reference to the 

 skeletal plates was correctly given by Biitschli (1885) and Bergh (1886). 

 but the number and position of the plates are not completely or correctly 

 given by either author. The plates composing the skeleton and the 

 nomenclature here used were described by me in a recent paper (1907 b). 



Formation of New Skeletal Parts. 



As the schizonts diverge after nuclear division, their exposed plasma 

 adjacent to the fission plane is moulded into the form of the completed 

 epitheca and hj'potheca and is coincidentlj' reclothed with a thin hyaline 

 pellicle, which in a short time shows the pores and ai-rangement of suture 

 lines and plates characteristic of the species. The newly formed skeletal 

 parts, as I have shown elsewhere (1908), take on the facies of the pa- 

 rental individual ; that is, if the parental skeleton was of a coarse-ribbed 

 and rugose type or of a more delicate and hyaline cast, the newly formed 

 plates of the daughter schizont are speedily thickened and their surface 



