112 THE DEVEI.OPMEX'I' 



distinct not mcreh- in position, but also in the time and manner 

 of development. 



Thus the stomodeum, or mouth, and the proctodeum, or anus, 

 are both formed at a very early i^eriod, and lined with epiblasf, 

 which gradually thickens, till under a low power it appears as a 

 very thick dark line. Certain involutions and almost invagina- 

 tions occur in the straight tube forming the mouth, which renders 

 it better suited as a prehensile organ. On March 29th and 30th 

 the lii)s are of a very simple and primitive character, the upper 

 lip being able to close over the under in an apparently close and 

 perfect gras]). But at this period it is well to remember that 

 unless the food were digested in the mouth cavity itself, it would 

 have been i)erfectly useless for the animal to have eaten, as none 

 of the products of digestion could jjossibly have found their way 

 by the usual channel through the body. From observations made 

 on the livingfcreatures, I am, however, of opinion that the animals 

 at this period did actually take some kind of material into their 

 mouths, as stated in the last pa])er ; but if so, any undigested 

 matter must have found its exit by the same way in which it 

 entered, thus showing a slight resemblance to the Coelenterata in 

 the mode of obtaining nourishment. 



This process, however, continues only for a very short period, 

 and although, of course, we cannot state that every animal 

 dissected on the same day is precisely in a similar phase of 

 development, yet from the number of dissections made and the 

 general similarity of the appearances found in all the sections 

 taken from various animals on the same day, it is certain that not 

 more than two days, and probably in many cases much less than 

 this time, elapses from the instant the creatures open their mouths, 

 and thus necessarily fill this portion of the alimentary canal with 

 water to the period when the middle portion of the canal, or true 

 stomach and int:estines, are fully formed so as to exhibit their 

 spiral formation. 



Directing our attention to the mouth only, we find that the 

 upper lip first becomes somewhat indented, the outer portion of 

 the indentation growing into a very delicate prehensile organ, 

 furnished at the extreme edge with a number of minute teeth or 

 bristles with recurved points, so small as to require fairly good 



