ABNOMALITIES IN CESTODE MONIEZIA. 155 



leading to the pore are undeveloped (Figs. 22, d, Ft. I. ; 39, b, 

 Pt. II., etc.), or else they open through another pore (Fig. 21, 

 a' , Pt. I. ; Fig. 40, b, Pt. II., etc.). There is in these cases no 

 indication of any morphological connection between the pore and 

 any other parts that may be present, but it may be that the pres- 

 ence of the internal reproductive organs favors in some manner 

 the formation of a pore (though not essential). While this con- 

 dition occurs so commonly, its complement, development of the 

 middle portions of the ducts without a pore at their outer end 

 or the internal organs at their inner end, has never been observed 

 by me. Thus there is apparently a certain tendency for the 

 organs in the central parenchyma and the marginal pore to ap- 

 pear, even without connection, while the middle portions of the 

 ducts never develop alone. In other words the central paren- 

 chyma and the margin are regions of " least resistance." I think 

 the absence of the middle portions of the ducts in so many cases 

 may be due in part to the fact that these portions traverse the 

 thick muscle layers which lie between the central and peripheral 

 parenchyma. The ducts, like the other portions of the organs, 

 appear first as aggregations of parenchyma-nuclei in a mass of 

 undifferentiated cytoplasm. Very few or none of these nuclei 

 exist in the muscle layers. Those portions of the ducts which 

 penetrate the muscles appear normally somewhat later than the 

 outer and inner parts, apparently because the material is less 

 abundant here and probably also because the muscle-layers offer 

 a certain degree of resistance to the penetration of the ducts. 

 Apparently either the outer or inner portions or both grow into 

 the muscle-layer and so become continuous. Now it is ex- 

 tremely probable that if any tendency to incomplete develop- 

 ment of the reproductive organs exists in the lateral regions it 

 will show itself first in the region where material is least abun- 

 dant and growth apparently most difficult, /. c., in that portion of 

 the ducts which lies in the muscle-layers. This portion might, 

 therefore, fail to appear, whereas the outer and inner parts, lying 

 in regions where material is more abundant and where no special 

 resistance to growth exists, probably do not require so strong a 

 stimulus for their development. 



The various abnormalities in the reproductive organs afford 



