INITIAL STAGES OF DEVELOPMENT. 721 



tionship of the worms. The differences evidently result from differ- 

 ences in the hosts, and similar instances are found in the case of 

 other parasites. The BotJiriocepJiali found normally in the dog are also 

 (according to Braun) far inferior to those of man, both in length and 

 thickness. The head only measures a third of the usual size, the 

 joints are smaller and flat, and in some cases thinner than in the 

 above-mentioned Bothriocephali from the cat. 



Satisfactory as are the conclusions to be drawn from such experi- 

 ments, our knowledge as to the actual progress of development is still 

 very incomplete. 



Only the processes of formation of the embryo are really known, 

 which have been recently elucidated by the work of Schauinsland 

 above referred to. 



We have already (p. 321) noted that the mature eggs of Bothrio- 

 cephalus latus include within the ovum proper a large quantity of 

 granular yolk-cells, and are surrounded by a hard shell 

 with an operculum. The germinal vesicle, hidden amid 

 the yolk mass, is rarely visible without special treat- 

 ment, at least after the shell has become quite brown. 

 When the eggs are laid they have generally already 

 passed through the first stages of development. Under 

 favourable circumstances, a clear spot can be seen FIG. 381. Ovum 

 through the shell amid the yolk spheres, which gradu- ^^h^yoik' 

 ally increases to form the rudiment of the embryo. cells and shell. ( x 

 The yolk-cells, as is well known, take no direct share 

 in the formation of the latter. They persist for a while unmodified, 

 and then, breaking up, are absorbed as nutritive material by the cells 

 of the embryo. 



The first changes in the ovum have not been observed. From 

 subsequent processes, however, they seem to consist in a segmentation 

 which after the first cleavage into two becomes unequal, resulting in 

 products which differ in position and appearance. The one set are 

 the true embryonic cells. At first, four in number, they occupy as 

 before the middle of the ovum, while the others, also few in number, 

 become separated off, pass through the surrounding yolk-cells to the 

 surface, and there form by flattening and peripheral coalescence a thin, 

 clear, enveloping membrane, which persists for a time, and is some- 

 times recognisable, along with the remnants of the yolk, even till the 

 liberation of the embryo. There can be little doubt that these en- 

 veloping cells represent the covering cells which have been already 

 mentioned in connection with the cystic tape-worms, and which van 

 Beneden 1 has since described in the development of the Tcenice as 



1 Archives de Bid., t. ii., p. 185, 1881. 

 2 Z 



