I 70 GA R DINER. [Vol. X I . 



Fig. 35, a horizontal section through a very advanced embryo. 

 This section suggests in its appearance a larval coelenterate 

 planula of some kind. From the remnants of this mesento- 

 dermal mass the parynchym of the body is formed. 



No trace of an alimentary tract is to be foimd at any time 

 during the development, for I have sectioned with great care 

 every stage up to and including the free-swimming young. 

 During the summer of 1894 I obtained the ova of the undescribed 

 species of ApJianostoma ( .-^ ) referred to above and compared its 

 segmentation with that of Polychoeriis. In every detail the 

 processes are alike in both : the formation of the eight cells 

 by the continuous division of the original macromeres ; the 

 subsequent division of each of these cells so as to form a thirty- 

 eight-celled stage ; the existence of a segmentation cavity and 

 its obliteration by the sinking in of the remnants of the two 

 original macromeres, and throughout all of this the mainte- 

 nance of the antero-posterior, as well as bilateral symmetry. 

 All of these points have been carefully observed and compared, 

 though no attempt was made to study the later stages of 

 ApJianostoma. 



The only work which I have seen which treats of the 

 development of any of the Acoela is a paper by Mile. S. 

 Pereyaslawzew (8) in which she describes the segmentation of 

 ApJianostoma, Nadina, Proporus, Convoluta, CyromorpJia. She 

 finds that in all of these ge7tera the segmentation process is 

 identical. Unfortunately no figures are given in her paper, 

 and it is impossible to follow the process from description 

 alone with absolute certainty. It is apparent, however, that 

 while my observations may differ from hers in a few minor 

 details, yet we agree as to the general plan of segmentation. ■ 



W. Repiachoff in a " Nachtrag zu Pereyaslawzew" states 

 that he also has studied the segmentation of the same form as 

 Mile. Pereyaslawzew described, and finds that his observations 

 with very slight exceptions confirm those made by her. He, 

 however, states that he has seen in the " Archigastrula ahn- 

 liches Stadium" " eine deutliche Urdarmhohle." By " Archi- 

 gastrula ahnliches Stadium " he probably refers to the stage 

 shown in Figs. 14, 15, 16, and Fig. 27, which latter is a longitu- 



