150 CHILD. 



the left end of each of the dorsal furrows is clearly due to a pro- 

 gressive reduction and final cessation of proglottidal growth in 

 the region under consideration. 



It is the left posterior dorsal portion of each proglottid which 

 has failed to develop fully in this case, while in Fig. / 3 it was the 

 right anterior ventral portion. If development in Fig. 74 were 

 complete enough to cause the formation of furrows all the way 

 to the margin a spiral would be formed opposite in direction to 

 that in Fig. 73, simply because of the difference in position and 

 relations of the imperfectly developed regions. But the furrows 

 on the dorsal surface curve to such an extent that they do not 

 reach the margin at all. On the ventral surface there is no re- 

 duction of the proglottids, but they are not differentiated at the 

 extreme margin. Consequently the left margin of the region 

 b-c-d is not divided into proglottidal regions. 



A knowledge of the arrangement of the transverse nerve-com- 

 missures in regions of spiral variations of this kind would be most 

 interesting and valuable. 



The spiral variations afford further illustrations of the inde- 

 pendence of the right and left proglottidal anlagen. In general, 

 the spirals are due to some irregularity in time of growth and 

 differentiation of parts, or to incomplete growth in either the right 

 or left half of the body. A variation of this kind having appeared 

 in a single proglottid shows a certain tendency to reappear on the 

 same side in a number of proglottids (c. g., Figs. 73 and 74). 



The condition and development of the furrows cannot be re- 

 garded as an exact indication of the conditions in the proglottids 

 which they bound, though a study of the arrangement of the 

 reproductive organs in the abnormal proglottids shows that the 

 furrows do indicate very closely the degree of development or 

 distinctness attained. 



But since the earliest stages in proglottid-formation occur only 

 in the central parenchyma, the formation of inter-proglottidal fur- 

 rows on the surface of the body being a later process, it is evi- 

 dent that a proglottid may attain a certain degree of development 

 without being represented by furrows on the surface. That such 

 cases do actually occur was pointed out in Part I. (c. g., Fig. 10, 

 b, p. 226 ; Fig. 20, /, g, pp. 240-241). In Figs. 10, b, and 20, 



