1907.] NATURAL SCIENCES OF PHILADELPHIA. 535 



cells in the entire embryo. So far all the cells of this quartet lie on the 

 surface of the ejip;. At the next division in this quartet however the 

 cells 2a'*-2d'' cut off a small cell to the exterior and the major portion 

 of the larger cell pushes inwards (figs. 30, 32), so that they become 

 almost covered by the surrounding cells. Unfortunately I have not 

 been able to follow the further divisions of these cells with certainty. 

 At a later stage one finds one or two more small cells lying over these 

 larger, deeper lying ones. For this reason I am led to suspect that these 

 cells bud off one or two more ectoderm cells. The major portions of 

 these four cells remain on the interior of the egg and form a portion of 

 the mesoderm. Since these cells when first budded in are well towards 

 the lower (oral) side of the egg, it is very probable that they form a 

 portion of the mesoderm around the blastopore. In later stages (PL 

 XL, figs. 36-39) a considerable amount of mesoderm is found in this 

 region. This later supplies the muscles and other mesodermal 

 structures connected with the pharynx. The other mesoderm cells 

 found just beneath the ectoderm in these stages are derived from the 

 divisions of -id, as will be descril)ed shortly. 



This account of the second quartet agrees closely in its main features 

 with that of Wilson (98) for Lcptoplana. Wilson, however, believed 

 that these second quartet cells, on the interior, multiplied rapidly and 

 formed the entire mesoderm of the body. The development of the 

 remainder of the mesoderm will })e dealt with farther on, and it need 

 only be pointed out here that this account confirms Wilson's with 

 regard to ectoderm arising from the second quartet. In contradiction 

 to Lang (84), who believes that the whole of the second and third 

 quartets formed mesoderm, we find here only a relatively small portion 

 of the second quartet budding to the interior. By far the greater bulk 

 of this quartet is ectodermal. 



With regard to the third ([uartet, this is in all i^robabilit}' entirely 

 ectodermal. The cells of this quartet when first formed are relatively 

 very small. These cells divide in a nearly radial direction (slightly 

 dexiotropic) at about the seventy-four-cell stage (fig. 26). Further 

 divisions of this quartet have not been traced accurately. At later 

 stages, however, one or both of these cells in each quadrant have 

 divided and all their progeny remain on the surface. There is no 

 indication that any of these cells pass to the interior. Their small 

 size and epithelial character indicate that they are purely ectodermal. 



History of the Fourth Quartet — The Mesoblast. 

 Shortly after the apical cells are formed and the cells 2a--2(P have 



