188 THE MICROSCOPE AND ITS REVELATIONS. 



' cephalic ' and the ' visceral ' portions of the mass (H) ; and the evolution 

 of the former into distinct organs very speedily commences. In the first 

 instance, a narrow transparent border is seen around the whole embryonic 

 mass, which is broader at the cephalic portion (i); next, this border is 

 fringed with short cilia, and the cephalic extension into two lobes begins 

 to show itself; and then between the lobes a large mouth is formed, open- 

 ing through a short, wide oesophagus, the interior of which is ciliated, 

 into the visceral cavity, occupied as yet only by the yolk-particles origi- 

 nally belonging to the ovum (K). 



584. Whilst these developmental changes are taking place in the em- 

 bryo, the whole aggregate of segments formed by the yolk-cleavage of 

 the infertile eggs coalesces into one mass, as shown at A, Fig. 405; and 

 the embryoes are often, in the first instance, so completely buried within 

 this, as only to be discoverable by tearing its portions asunder: but some 

 of them may commonly be found upon its exterior; and those contained 

 in one capsule very commonly exhibit the different stages of development 

 represented in Fig. 404, H-K. After a short time, however, ib becomes 

 apparent that the most advanced embryoes are beginning to swalloiv the 

 yolk-segments of the conglomerate mass; and capsules will not unfre- 

 quently be met-with, in which embryoes of various sizes, as a, 1), c, d, e 

 (Fig. 405, A), are projecting from its surface, their difference of size not 

 being accompanied by advance in development, but merely depending 

 upon the amount of this * supplemental ' yolk which the embryoes have 

 respectively gulped-down. For during the time in which they are engaged 

 in appropriating this additional supply of nutriment, although they in- 

 crease in size, yet they scarcely exhibit any other change; so that the large 

 embryo, Fig. 405, e, is not apparently more advanced as regards the for- 

 mation of its organs, than the small embryo, Fig. 404, K. So soon as 

 this operation has been completed, however, and the embryo has attained 

 its full bulk, the evolution of its organs takes-place very rapidly; the cili- 

 ated lobes are much more highly developed, being extended in a long 

 sinuous margin, so as almost to remind the observer of the ' wheels' of 

 Rotifera ( 445), and being furnished with very long cilia (Fig. 405, B); 

 the auditory vesicles, the tentacula, the eyes, and the foot, successively 

 make their appearance; a curious rhythmically-contractile vesicle is seen, 

 just beneath the edge of the shell in the region of the neck, which may, 

 perhaps, serve as a temporary heart; a little later, the real heart may be 

 seen pulsating beneath the dorsal part of the shell; and the mass of yolk- 

 segments of which the body is made-up, gradually shapes itself into the 

 various organs of digestion, respiration, etc., during the evolution of 

 which (and while they are as yet far from complete) the capsule thins- 

 away at its summit, and the embiyoes make their escape from it. 1 



585. It happens not unfrequently that one of the embryoes which a 

 capsule contains does not acquire its ' supplemental ' yolk in the manner 

 now described, and can only proceed in its development as far as its ori- 

 ginal yolk will afford it material; and thus, at the time when the other 

 embryoes have attained their full size and maturity, a strange-looking 



1 The Author thinks it worth while to mention the method which he has found 

 most convenient for examining the contents of the egg-capsules of Purpura; as 

 he believes that it may be advantageously adopted in many other cases. This 

 consists in cutting off the two ends of the capsule (taking care not to cut far 

 into its cavity), and in then forcing a jet of water through it, by inserting the end 

 of a fine-pointed syringe ( 127) into one of the orifices thus made, so as to drive 

 the contents of the capsule before it through the other. These should be received 

 into a shallow cell, and tirst examined under the Simple Microscope. 



