16 DISCOPHOR^. Part III. 



The first indication of any change taking place in the interior of these ciUated 

 glohes is a growing transparency of the peripheric portion, jnst beneath the coating 

 of ciha {Fir/. 25), and also a similar modification of the centre of the mass. Soon 

 the nature of these changes becomes more obvious, as we find that the outer portion 

 of the embryo grows more and more transparent, until a distinct layer {Fig. 26 a) 

 declares itself, surroundmg the whole mass as if with a thick envelope. At the 

 same time the centre continues to increase in transparency over a larger field, 

 initil the whole is lighted up as if hj an interior illumination. By plunging the 

 focus of the microscope to the centre of the embryo, we find there a spherical 

 cavity {Fig. 27 d) with a very clearly marked outline. This at once gives a defi- 

 nite character to the different regions of the body: the outer envelope is the 

 outer wall {a) of the body, the part included by this is the inner wall {h), and 

 the cavity {d) is the digestive cavity in an incipient state. As yet there is 

 nothing present which indicates either right and left or before and behind, but 

 every thing is equally disposed about a central spherical cavity. The average 

 diameter of the majority of the embryos at this time is g^^ of an inch : some, 

 however, measure as small as 3^3 of an inch, and others as large as ^is of an 

 inch. The digestive cavity continues to enlarge until its diameter is equal to half 

 that of the whole body {Fig. 28 d) before any other sensible changes take place. 

 Up to this time the embryo has been of a uniform, transparent gray color; but 

 now the inner surface of the digestive cavity {Fig. 28 d) is tinted Avith a faint 

 rosy color, which suffuses the whole body with a delicate blush. 



The next phase introduces the formation of the mouth. This is brought about 

 in the first place by the formation of a depression {Fig. 29 e) on the outer surface 

 of the inner wall {b), and from thence a passage is formed inwardly to the digestive 

 cavity {d). The outer wall is pierced, sometimes soon and at other times much 

 later. After the formation of the mouth and the passage-way to the digestive 

 cavity they are seldom seen, becavise the embryo keeps them closed, except Avhen 

 swallowing its food; and hence some of the older forms figured on this plate 

 appear to have no mouth {Figs. 31 and 32), or no passage {Figs. 30, 34, 35, and 

 36) to the interior. The figure which we have referred to for the formation of 

 the mouth and the passage-way to the digestive cavity {Fig. 29) was contracted 

 vertically at the moment it was drawn, but the true form is oval like the figure 

 below it {Fig. 32). The degree of contractility which these embryos possess is 

 well illustrated by two figures {Figs. 31 and 32) placed here side by side; for 

 these figures were copied from the same individual. This faculty is possessed by 

 the embryo from the earliest period after segmentation has finished, and increases in 

 degree with the development of the body. Sometimes one may observe a single 

 organ or part of the body contract or expand, while the rest remains immovable ; 



