80 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



FORMATION OF THE MESODERM. 



There is considerable difference of opinion among those who have investigated the embry- 

 ology of Phorojiis as to the origin of the mesoderm, and there seem to be no two whose descrip- 

 tions agree, although Ikeda (9) and Longchamps (12), in their recent papers, arrive at the same 

 conclusions, generally speaking. 



The study of the eggs and larva' of Phoronis architecta and those of Phoronis australis 

 show that the great difference in the origin of the mesoderm, as Roule (20) and Masterman (16) 

 sec it, may lie due. in a great part, to difference in the larvae themselves. 



The eggs and embryos of Phoronis mi si rolls, for which we are indebted to Mr. Ikeda, 

 arc very similar in appearance to those of Phoronis buskii, judging from Masterman's figures 

 (l(i). Sections of the eggs and larvae of the former show the development to be of the same 

 general type as that of J 'homo Is ljlno/1. which Ikeda (9) has described. 



The eggs and larva? of Phoronis architecta are considerably different from those mentioned 

 above. They are more regular in form, the blastoccele is much more spacious and the cells 

 themselves are more regular in shape and arrangement. They are most similar in appearance to 

 the early stages of Phoronis sahatieri studied and figured by Roule (20) and those of "Phoronis 

 oV Helgoland" figured by Longchamps (12). 



The formation of the mesoderm begins in Phoronis architecta as soon as the flattened side of 

 the blastula begins to gastrulate. In a few cases round blastulee are found, within the blastoccele 

 of which are rather large granular spherical bodies much larger than the plasmic corpuscles 

 described above. Each of these contains an opaque body which takes saffranin stain with readi- 

 ness. A comparison of these bodies with the nuclei of the cells of the blastula wall convinces 

 one at once that they are not nuclei. In tig. 1-ta a section through such a blastula is shown in 

 which these bodies are seen within the wall of the blastula as w T ell as inside the blastoccele. They 

 are embedded in the wall without reference to the limits of the cell and usually occupy the width 

 of two cells. The cells inclosing these peculiar bodies do not differ from the cells surrounding 

 them in that region and each has its own nucleus. These bodies are not the cut ends of amoeboid 

 processes which Caldwell (3a) and Roule (20) observed, for such processes do not occur in the 

 blastuhe of Phoronis architecta. We are unable to make any positive statement as to their fate, 

 but it is very probable that they break up into the smaller plasmic corpuscles. Such bodies as 

 the former might easily be mistaken for mesoderm cells, and we suspect that the "mesoderm 

 cells" observed by Foettinger (5), Metschnikoff (18), and E. Schultz (21) in the round blastula 1 

 were of the same character. 



The work on Phoronis architecta indicates that the mesoderm which forms the lining of the 

 preoral lobe and the collar cavities of the Actinotrocha arises from the lips of the blastopore. 

 As to the origin of the lining of the trunk segment, we are still in some doubt, but we are 

 inclined toward Longchamps's suggestion that some of the cells of the nephridial pit give rise to 

 it. Caldwell (3a) also holds that the mesoderm arises from the endoderm, assuming that the 

 "posterior pit" (nephridial diverticulum), which he considers to be one point of origin of the 

 mesoderm, is of endodermal origin. Roule (20) derives most of the mesoderm from the endo- 

 derm, but also considers the " bandelettes mesoltlastiques," which Schultz (21) first pointed out to 

 be the same as the posterior diverticulum of Caldwell, as giving rise to mesoderm. 



As is seen on referring to tig. 15, the flattened part of the wall of the blastula has become 

 more than one cell thick. In fact active cell division lias taken place. Yet most of these cells 

 are destined to become the wall of the archenteron, and only a few are to give rise to mesoderm. 

 Careful examination of many sections fails to show that mesoderm cells ever have their origin 

 from the dorsal surface of the archenteron. In this respect the development of Phoronis archi- 

 tecta seems to agree with that of Phoronis Icowal&oskii as described by Caldwell (3a) and Long- 

 champs (12). and that of Phoronis buskii, which Masterman (1(3) investigated. In Phoronis 

 architecta, as in the form studied by Longchamps (12), the anterior and lateral borders of the 

 blastopore are most active in giving rise to mesoderm (tigs. 16 <i, b, c, d, e,f). Most of it is 



