172 



greater area along the future ventral and posterior regions, as is seen 

 below in fig. 2B, of this paper which is a copy of Ijima's figure 28, 

 Plate XXIII. This more extensive thickening begins in D. lacteum and 

 other planarians (see figure 2) about the time that the first anläge of 

 the adult pharynx becomes evident. In view of this extension of the 

 mesodermal thickenings it seemed to me, when I found in Planaria nia- 

 eulata the adult pharynx not originating in the place previously occu- 

 pied by the embryonic pharynx, that the most probable explanation of 

 Ijima's account was not a difference in orientation between two such 

 closely related forms, but rather a mistake on the i)art of this author 

 which arose from the fact that the two structures were never present 

 together in the embryo of I), lacteum. It seemed to me that if the point 

 at which the last trace of the temporary pharpix disappeared was not 

 quite so near the region of the adult structure's origin as Ijima sup- 

 posed, i. e. if it was located a short distance up along the ventro-posterior 

 surface of the body as shown in figure 2 ^, C and E, then the case of 

 D. lacteitm would not be radically different from the condition which 

 was so clearly to be made out in P. macidata. Accordingly I suggested 

 this in my description of these organs in that species, not as a criticism 

 of Ijima's conclusion, but merely to j)oint out an error into which it 

 seemed likely he had fallen through the difficult nature of the material. 

 In view of the care with which Matties en has followed out all of his 

 work I do not for a monent doubt the absolute correctness of his state- 

 ment that the adult pharynx of P. torva appears immediately posterior 

 to the degenerating embryonic structure. Now that this is established 

 we may I think believe that Ijima was quite right in his orientation 

 of these organs in D. lacteum. But I think it is now equally well estab- 

 lished that the same relation does not exist in P. maculata and probably 

 in another American form P. simplicissima. Since Matti e sen seems 

 unwilling to accept my description as reliable I will say this regarding 

 the basis upon which it rests. In the first place the account was not based 

 upon any single but upon a number of embryos, and moreover I have 

 carefully reexamined all my preparations without finding anything 

 which would lead me to change my account. Second, it has been con- 

 firmed by Bardeen'' who discovered it independently but shortly after 

 I did. For comparison with my own figure I have reproduced (fig. 2 Ä] 

 the outlines of Bardeen's figure of this stage, and quote the sentence 

 in which he refers to this. On page 263 he says: ,,The hitherto sj)herical 

 embi'yo now becomes flattened dorso-ventrally and elongated in the an- 



"^ C. E,. Bardeen, Embryonic and Regenerative Development in Planarians. 

 Biological Bulletin Vol. III. No. 6. Nov. 1902. 



