80 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 4. 



FORMATION OF THE MESODERM. 



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

 ology of Phoronis 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 larvae of Phoronis architecta and those of Phoronis austral!* 

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

 see it, may be due, in a great part, to difference in the larva? themselves. 



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

 are very similar in appearance to those of Phoronis husk!!, judging from Masterman's figures 

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

 general type as that of Phoronis !jlmal. which Ikeda (9) has described. 



The eggs and larvae of Phoronis a/rchitecta are considerably different from those mentioned 

 above. They arc more regular in form, the blastocoele 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 sabatieri studied and figured by Roule (20) and those of u Phoronis 

 d' 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 blastuhe are found, within the blastocoele 

 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 satfranin 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. l-ia a section through such a blastula is shown in 

 which these bodies are seen within the wall of the blastula as well as inside the blastocoele. 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 

 blastula? 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 blastuhe 

 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 lie 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 mesoblastiques," 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 has 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 kowalt vsMi as described by Caldwell (3a) and Long- 

 champs (12). and that of Phoronis host,-;;, which Masterman (16) 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 tf, 6, c, d, e,f). Most of it is 



