78 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Caldwell (3) says that in the four-cell stage two smaller clear and two larger opaque cells are 

 present. Masterman ( L6) finds that in the four-cell stage the blastomeres taper toward one pole, 

 and that this results, when the third furrow appears, in the upper four being less in hulk than 

 the lower four. Masterman's description, 1 find, applies to the eggs of Phoronis amtralis. 

 Ikeda (9) did not discover any appreciable difference in the size of the blastomeres until the 

 eight-cell stage. At this time, he says, "it will be seen that the upper four blastomeres are 

 very slightly smaller than the lower four." 



In Phoronis architecta the first cleavage plane is meridional and usually divides the egg into 

 two practically equal blastomeres (tig. 3), although sometimes the division is decidedly unequal 

 (tig. 4). The cleavage furrow begins in the region of the polar bodies (tig. 5). After the com- 

 pletion of the first cleavage and sometimes before, the first polar body divides (fig. 6). In fig. 6 

 is seen the reconstruction of the nuclei after the first cleavage. Immediately before the second 

 cleavage the two blastomeres, which were closely applied to one another after the first cleavage, 

 come to overlap. About fifteen minutes after the first cleavage the second cleavage takes place. 

 It is meridional and at right angles to the first, dividing the two equal blastomeres into four 

 equal blastomeres. As Ikeda has observed, the cleavage does not occur simultaneously in both 

 blastomeres nor does it in later cleavages (tig. 7). The blastomeres of the four-cell stage which 

 at first overlapped soon become applied to one another so that the two meet in a cross furrow 

 (fig. 8). Shortly before the third cleavage occurs the cross furrow disappears and the blasto- 

 meres come to overlap. The third cleavage takes place fifteen minutes after the second cleavage, 

 and it is equatorial. The blastomeres become drawn out into a more or less ovoid shape and, as 

 division takes place, the upper four blastomeres become rotated in the direction of the hands of 

 a watch (tig. !). The eight blastomeres are approximately the same size, as a rule, and there is a 

 small segmentation cavity present which from now on persists (tig. 10). The three polar bodies 

 arc distinguishable at this stage sometimes within the blastoeoele and sometimes on the surface 

 of the blastomeres. The blastoeoele is open at the animal and vegetal poles. The sixteen-cell 

 stage arises from the eight-cell stage by a meridional division of each of its blastomeres. but 

 they do not all divide simultaneously (tig. II). although the difference in time is very slight. 

 After the sixteen-cell stage the individual blastomeres were not followed. The division takes 

 place rather irregularly, but the blastomeres are all about of the same size. 



The so-called 'blastoeoele pore," observed by Ikeda (9), was found occasionally in young 

 blastuhe, but it does not seem to be of constant occurrence nor definite in position {dgx. 12, 13). 



Two hours after the first cleavage the blastula is composed of seventy or eighty cells, and it 

 is still inclosed in the ego membrane. Four hours later the membrane disappears, and the 

 ciliated blastula begins to swim (tig. 11). 



The blastomeres were so much alike and so uniform in size that their individual history was 

 not traced. It would seem probable from Masterman's work on Phoronis bushii (16) that the 

 cell lineage might be followed in that form, for he finds considerable difference in the size of 

 the blastomeres in the early stages of cleavage at least. 



The apical pole of the ciliated blastula is provided with long cilia (tig. 14). The nuclei are 

 situated nearer the outer than the inner surface, and the inner ends of the cells are filled with 

 rather dens,, granules. In the segmentation cavity are found the so-called "corpuscles." which 

 have been observed by most investigators working on the early stages of Phoronis. Caldwell's 

 (3a) view that they are not mesoderm cells is undoubtedly correct. Our observations agree with 

 those of Ikeda (!)), for the "plasmic corpuscles" are much smaller than any of the cells of the 

 blastula. and none with nuclei were found. In Phoronis architecta they do not appear until the 

 late blastula stage (tig. 11), at which time the inner ends of the cells are densely granular. It 

 seems very probable that the corpuscles are pushed out from the densely granular part of the 

 cell, and that, as Caldwell and Ikeda have held, they are an extra supply of nourishment. 



The blastuhe. gastruhe. and young larva' of Phoronis architecta are quite similar in appear- 

 ance to those of I 'Imr, mis d? Helgoland which Longchamps (12) has figured. The development is 

 more regular than that of most other species, which is probably due to the fact that the eggs 

 and embryos are not harbored in the tentacular crown. 



