712 EMBRYOLOGY. 



and 37. In the profile (Fir/. 36) we notice the same tendency in the digestive 

 cavity (d) to incline towards the lower side, after the dorsal portion has in- 

 creased more rapidly, giving the anal part ol" the embryo a bevelled appear- 

 ance. In a somewhat older stage ( /■'///• 38) the digestive cavity is still longer. 

 and almost touches the lower side. We notice a difference between the star- 

 fish and the sea-urchin in the time of formation of the alimentary canal, the 

 stomach, and the oesophagus. In the starfish the mouth is formed before 

 the differentiation of these organs takes place ; while in the sea-urchin the 

 mouth is not formed until the alimentary canal and the oesophagus, as well 

 as the water-tubes, are quite distinctly defined (see Fig, ;.;). What is 

 also peculiar to Echini is the presence of large masses of yolk-cells along the 

 sides ol' the digestive cavity, indicative of the great changes which take place 

 at the points where these masses of yolk-cells are most numerous. We have 

 observed that the yolk-cells are always present wherever any new organ is 

 developed ; in these embryos the appearance of the water-tubes is preceded 

 by an accumulation of yolk-masses at the extremity of the digestive ca\ii\ 

 (see Fig. 38), and the place of the limestone rods (Fig. .;/'./•') is lirst seen 

 filled by clusters of yolk-masses, in the midst of which the rods are deposited. 

 Kods extending into the a mis are characteristic of Echinoids and Ophiurans : 

 we find nothing of the kind in starfish or Holothurian embryos. 



In the next stage (Fig. 39) the original cylindrical digestive cavity has 

 already a decided tendency to differentiation, the walls of the stomach (d) 

 and of the (esophagus [o) being of very different thickness; from o, the 

 pouches which are to become the water-tubes {"■, «/) project far beyond the 

 outline of the digestive cavity. The limestone rods {/./) can faintly he dis- 

 tinguished from the mass of yolk-cells which surround them. The cal- 

 careous cells, which take such a great prominence in older embryos (sec Fig. 

 .:;). make their appearance as early as the stage ol' Fig. -is; they are quite 

 large in the condition represented in Fig.  ','.<. The marked contrast which 

 already exists between the different parts of the digestive cavity is still more 

 apparent in a stage hut slightly more advanced (Fig. ',")■ The dorsal portion 

 of the embryo has up to this time been growing most rapidly, changing the 

 outline of the pluteus. particularly when seen in profile : in the subsequent 

 figures the outline, when seen from above, is also undergoing great changes. 

 The pluteus assumes a more rectangular shape ( Fig. ;/ ) when seen either from 

 above or from below. The water-tubes are almost separated from the diges- 

 tive cavity, which has been divided into three very distinct regions (c, d. <>. 



