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



While the above changes have been taking place in the preoral end of the larva there has 

 also been some change in the postoral region. It is seen from tigs. 16 c, d, e, f that in the 

 young gastrula with the large circular blastopore mesoderm cells are being pushed out into 

 the blastocoele along both sides of the archenteric wall back almost to the posterior border of 

 the blastopore. At this stage large spherical cells with rather small, deeply staining nuclei 

 are sometimes seen floating freely in the blastocoele (tig. 17'/). These cells have their origin 

 in the wall of the archenteron (fig. 17) and are quite different from the bodies found in the 

 blastocoele of the blastula. They have a definite nucleus and they seem to be similar cells to 

 those found by Ikeda in the larva with one pair of tentacles. They certainly do resemble the 

 blood corpuscles found in the older larvae, only they are considerably larger. Ikeda (9) came to 

 the conclusion that these cells were the " mother cells of blood corpuscles which are found as 

 corpuscle masses in the collar cavity of the Actinotrocha." Since the publication of his paper 

 Mr. Ikeda has written that he considers his theory concerning the fate of these cells to be 

 incorrect. They are easily distinguishable from all other cells by the fact that the}' are larger 

 and that the cytoplasm does not stain. They have a nucleus which is rather small. We shall 

 return to a consideration of these cells when we describe the blood corpuscles of the Actinotrocha. 



As the blastopore lips begin to close up posteriorly (figs. 18 d, e) the endoderm cells in that 

 region lose the power of giving rise to mesoderm cells, but they are still found arising in a more 

 anterior region. 



At a little later stage, in which the blastopore has become circular again after the fusion of 

 the blastopore lips and the enteron has almost reached the posterior end, a few mesoderm cells 

 are seen lining the ventral ectoderm in the posterior region (tig. 20^). These cells, however, 

 do not have their origin from the wall of the posterior part of the enteron nor from the ventral 

 ectoderm which Caldwell (3a) would call the "primitive streak." The cells forming the ventral 

 ectoderm are very regularly arranged into a layer one cell thick and all the nuclei are in a resting 

 state. The mesoderm cells have either migrated from the cells of the lateral cords which arc 

 prolongations of the sac, forming the lining of the preoral lobe (tig. 20e), or from the region of 

 the blastopore, where some mesoderm cells are still arising. In general, our interpretation of 

 the faets bearing on the origin of the mesoderm in the posterior region of the larva agrees with 

 that of Longchamps (12) for Phoronix TcowdU vskii. 



When the larva reaches the stage shown in fig. 21 where the blastopore is transverse and 

 the archenteron fuzes with the posterior ectoderm, the mesoderm cells are found to be more 

 numerous in the posterior region, and in nearly all cases they are applied to the ventral suface 

 of the blastocoele. At this time the proliferation of mesoderm cells from the endoderm has 

 ceased in the anterior region ami there is no indication of any mesoderm cells being given off 

 from most of the posterior region. At the extreme posterior end of the enteron, however, a 

 transverse section across the larva (fig. 22</) shows a mass of cells which might be taken for 

 proliferating mesoderm cells. Traced farther back, this mass of cells is found to be part of the 

 wall of the ••posterior pit," or. as Ikeda (!>) has called it, "the nephridial pit" (figs. 22 e,f, g). 

 The fate of the cells of the nephridial pit will be discussed in the description of the larva with 

 two tentacles. 



Larvae like the one just described do not show the least trace of a mesentery between the 

 collar and trunk. In fact, one could hardly say that a trunk existed at this time. The oblique 

 strongly ciliated tract of ectoderm which indicates the line of origin of the larval tentacles has 

 not appeared. 



FURTHER GROWTH OF THE YOUNG LARVA. 



The flexure of the preoral lobe continues as the larva grows older (fig. 2-1). In this way a 

 vestibule is formed and the original blastopore becomes the part which connects the vestibule 

 and archenteron (fig. 24). This relation between the vestibule and blastopore has been recog- 

 nized by Masterman (16), Roule (2<>). Ikeda (9), and Longchamps (12). Longchamps speaks of it 

 as a "stomodanim," and Masterman does also, but the latter adds " oesophagus " after it. (If our 



