242 EMBRYOLOGY. 



to develop into the fully formed animal, are much more numerous than before, 

 and are as yet without signs of differentiation into the fundaments of the various 

 organs. This statement may perhaps be modified in one particular, for on the 

 ventral side immediately posterior to the large test cells is a group of several 

 elongated cells extending from the surface to a considerable distance into the 

 interior. Their external position is marked by a shallow depression (Plate 14, 

 fig. 9), and appearances suggest that this is the region of the blastopore, and 

 that the cells are stomodeal elements destined to become much more numerous 

 and prominent in later stages. As may be seen in the figures no definite blasto- 

 cele is evident. 



Later stages are ushered in with the development of the test (Plate 13, fig. 

 11), the great increase in the number of the remaining ectoderm cells, and the 

 appearance of a definite stomodaeum, mid-gut, and cerebral gangUa together 

 with the appearance of recognizable mesodermal elements. In other words, 

 differentiation has now advanced to a stage where the three germ layers are 

 clearly defined, and certain systems broadly outlined so that the following account 

 will probably gain in clearness if these various systems are considered individually 

 rather than parts of the whole. 



The Test: — As has been noted some of the test cells have been seen to 

 arise at an early stage by tangential divisions of the relatively few cells com- 

 posing the body of the embryo. Concerning the cleavages of cells left in the 

 interior there is nothing known. The differentiation of the test itself from the 

 remainder of the ectoderm of the trunk region is accompUshed by a slower 

 rate of division that is soon brought to a complete standstill. As may be seen 

 in the figures (Plate 13, fig. 11, Plate 14, fig. 3), the test extends at the outset 

 farther over the ventral side than on the dorsal, but in the latest stage in the 

 present collection (Plate 14, fig. 4) a shifting has evidently occurred as the 

 brain is placed at the anterior end of the embryo, and the mouth is well forward 

 on the ventral side. Even yet the organ is asymmetrical in position, but more 

 radially adjusted than at first. Judging from the amount of yolk, contained in 

 the test cells of the oldest embryo it is evident that a very considerable time must 

 elapse before the nutritive material is absorbed and the remnants cast off. 

 While the nuclei present an irregular, somewhat shrunken appearance the cyto- 

 plasm is not vacuolated, as are functional test cells of Chitons or Yoldia for 

 example, and it is probable that a much greater diminution in size occurs before 

 these elements become wholly non-functional and worthless. 



That a diminution in the size of the test has already ensued in the oldest 



