STUDIES ON ASPLANCHNIA EBBESBORNII. 2OI 



play a very important role in the future development of the 

 individual. The derivatives of E give rise to most of the diges- 

 tive tract, the reproductive system and a few muscle fibers. 

 All other structures are derived from the surface cells, which 

 form a rather deep epithelial layer around the central cell E. 



Diagrams A-C, 6, represent late cleavage stages in which E 

 has passed into the interior of the embryo and is no longer visible 

 from the exterior. A little later stage (A-C, 7) than the pre- 

 ceding, shows the second cleavage of E as represented in an 

 optical section from dorsal view. Diagrams A-C, 8, represent 

 an embryo of either sex in optical section, as viewed from the 

 left side with five large central cells derived from E. The small 

 stippled space between the large cells and the outer epithelium 

 represents the first indication of the presence of the future body 

 cavity. 



C. DIFFERENTIATION OF THE CENTRAL CELLS. 

 The large central cells E 2 - 1 - 1 and E 2 - 1 - 2 give rise to the reproduc- 

 tive organs, and the derivatives of the large central cells E IA , 

 E--- A and E 2 --- 2 produce most of the digestive system. The large 

 central cells now divide very rapidly and fill the interior of the 

 embryo (A-C, 9). These central cells as stated above become 

 differentiated into two distinct regions, the entodermal (st) and 

 the reproductive portion (r.o.), which shows the presence of 

 darker granules and later gives rise to the reproductive organs 

 proper. A few muscle fibers are derived from the central mass 

 of cells, which are directly concerned in the movements of the 

 reproductive organs and the stomach w T ith the d igestive glands 



D. EMBRYONIC FOOT AND STOMOD.*;UM. 



The slight swelling at the lower posterior end (A-C, 9 ft.) 

 marks the beginning of the foot, an embryonic structure. Later 

 it becomes well developed in both sexes and shows the presence 

 of two distinct toes, which no doubt persisted as a functional 

 structure in its ancestors. The foot completely disappears before 

 the birth of the individual. Diagrams A-C, 10 show the early 

 stages in the formation of the stomodseum (sto.). The embryo is 

 considerably curved upon itself and occasions the close proximity 



