THE DEVELOPMENT OF FASCIOLAKIA. 141 



that the only method of determining the average number per 

 capsule was by finding the average number of larvae in the later 

 stages. The number of fertilized eggs in each capsule cannot 

 have been less than the number of larva; that come from it. 

 There are reasons moreover, such as the frequency of accidents 

 and the occurrence of dwarfed embryos, easily overlooked, 

 for believing that the number of fertilized eggs exceeds the 

 number of young found in later stages. The contents of 145 

 advanced capsules showed 6.2 larvae in each as an average 

 between a maximum of I 5 and a minimum of 2. Between these 

 extremes all degrees of fertility occurred. 



The stages of development found in a single capsule are as 

 variable as the number of larvae. Two cell stages may occur in 

 the same case with advanced embryos. It is difficult therefore 

 to form an idea of the rate at which an individual egg develops, 

 particularly as development ceases shortly after the embryos are 

 removed from the capsules. Judging from the great variety of 

 stages found in a single egg-case one may conclude either that 

 not all the eggs are fertilized in the oviduct, but that some are 

 impregnated after the capsules have been deposited, or that for 

 some reason certain ones undergo a longer resting period than 

 others. The importance of these discrepancies at the beginning 

 of development becomes apparent in later stages. 



After the early developmental processes have been passed 

 through a larva results so irregular that no two individuals of 



o * * 



this age are alike. Fig. 2 omits two eggs which this larva had 

 swallowed, but will serve very well without these to give an 

 idea of the general external appearance of the embryo before the 

 crisis of its larval life has occurred. The larva is represented as 

 viewed from the ventral surface. Anteriorly is the head vesicle 

 (/i.i'.] and posteriorly the body vesicle (.?'.). Between the two, 

 under the right external kidney (ex.k.\ two of the yolk spheres, 

 derived from the four macromeres of the segmentation period, 

 can be seen. The ectoderm of the body and head vesicles lacks 

 definite cell boundaries, though the indefiniteness is much greater 

 in the anterior than in the posterior region. Each nucleus of the 

 ectoderm stains deeply and is surrounded by vacuoles which 

 decrease in size inversely as their distance from it. Those 



