110 ISABELLA M. DRUMMOND. 



the left ventral wall of the stomach. Below the rectum, tiud 

 lying between it and the liver, is a little dense mass of 

 mesoderm cells, which is just beginning to be hollowed out 

 on either side to form the rudiment of the pericardium. Its 

 position is shown in the figure at p. c. The otocysts have 

 appeared on either side at o. t., and are still widely open to 

 the surface of the bod}'. 



Stage C (figs. C and Ci, and fig. 11). — Considerable 

 growth in length has taken place, and the different regions 

 of the body are clearly marked out. The foot is now a 

 prominent organ in the anterior ventral region, and a slight 

 constriction of the body separates the foot and head from the 

 now well-developed visceral hump. This latter is surrounded 

 posteriorly by the mantle folds (in./.), which form a 

 prominent ridge dorsally to the anus. At this stage the 

 first rudiments of the mantle cavity appear as two little 

 depressions lying one on either side of the anus. These are 

 best seen in a ventral view of the whole animal, or in section. 

 Fig. Ci is a ventral view of a slightly older embryo than 

 fig. C, but the essential relations of tbe organs are precisely 

 similar. Here the two depressions are seen at c. m. c. and 

 r. m. c, the right one being considerably in advance of the 

 left. The same depressions are seen in transverse section in 

 fig. 11. Von Erlanger describes the first appearance of the 

 mantle cavity as " eine kleine Grube" ventrally and just in 

 front of tbe anus, but it is quite clear that at this stage there 

 is a distinct rudiment on either side, the rectum passing 

 down a ridge between them [x in fig. 11), to open directly on 

 to the surface of the bodv. It is onlv at a later stage that 

 the portion of the body immediately in front of the anus 

 sinks in and unites the two original depressions, thereby in- 

 cluding the anus within the mantle cavity. I have never 

 been able to find a stage in which these two original depres- 

 sions are symmetrical. If this stnge closely corresponds, as 

 I believe it does, with von Erlanger's fig. 1, plate xxi, he 

 has overlooked an important point in the external anatomy of 

 the embryo. It is not, as he says, perfectly symmetrical 



