290 THE EMBRYOLOGY 



centre of the body (III, dc) ; next, the body continuing to in- 

 crease in size, and the indentation enlarging at a corresponding 

 rate, until it becomes a great cavity, the edge of the indentation 

 appears to be folded or rolled inwards (IV, m*) ; and finally, we 

 find this incurved edge extending (V, m l ) half-way to the bottom 

 of the cavity (dc) so as to form a sort of passage-way from the 

 latter to the outside, and at the exterior end of this passage- 

 way several little protuberances (t) arranged about it in a single 

 row, whilst within the newly formed cavity (dc) there may be 

 seen several low ridges (p) which divide the space immediately 

 about the passage-way into as many little stalls. At this latter 

 stage of development we may recognize the characters by which 

 the embryo can be identified as one of the Polyps, so that if we 

 were not cognizant of its origin there could be no hesitation as 

 to its typical relationship. For instance, that which I have called 

 the passage-way corresponds to the stomach, and the cavity, 

 (V, dc,) which commenced as an indentation, is the digestive 

 cavity, whilst the low ridges (p) in the latter indicate the position 

 of the primary set of serni-partitions, and the protuberances (t) 

 around the exterior opening, or mouth, of the stomach stand in 

 the place of the first group of tentacles. In the next and last 

 stage (VI) which I shall illustrate, the organs have assumed 

 very nearly the form and proportions of the younger phases, 

 which I have described in former lectures (pp. 59, 177) ; thus, 

 the tentacles (t) are prominent finger-shaped bodies ; the stomach 

 (st) is narrowed in front, where the mouth (m) is, and behind, 

 at the entrance (m l ) to the digestive cavity (dc) ; and the semi- 

 partitions are distinct membranes (p) which extend nearly the 

 whole length of the body and divide the space about the stom- 

 ach into totally separate apartments, whose only means of com- 

 munication with the rest of the body-cavity are through the ends 

 which lie next the posterior part of the embryo. 



Echinoder mala. I shall go no farther with the illustration of 

 the mode of development of the group of Echinoderms than is 

 sufficient to show that the various organs originate only at such 

 points as have the same relations to each other that we find in 



