1904.] NATURAL SCIENCES OF PHILADELPHIA. 343 



anteriorly which He near the enteron and are probably concerned in 

 the formation of splanchnic musculature. Similar conditions are found 

 to exist in Dreissensia, according to Meissenheimer (1901). 



Among the Pulmonates the work of Rabl (1879) is confirmed by 

 Holmes (1900), who finds that all the derivatives of the primary meso- 

 blast are mesoblastic in fate. More particularly he states that the 

 two bilaterally placed teloblasts give rise to a pair of small cells ante- 

 riorly, after which the large cells divide into equal moieties. Wier- 

 zejski (1897) says of Physa fortinalis, "Dass der Modus der Bildung 

 eines Theiles des Mesoderm bei Physa, desjenigen aus der Urmesoderm- 

 Zellen fast ganz derselbe ist wie ihn Heymons fiir Umbrella eingehenden 

 dargestellt". In the last stage described the mesoderm consists of 

 twelve cells, a group of six small cells anteriorly placed, behind which 

 are a pair of "Urmesoderm-Zellen " from which they arose, while behind 

 and above lie two other rather large mesoderm cells which have given 

 off a pair of small cells posteriorly. Both in sequence of origin, in 

 relative position and in size this group corresponds to the similar 

 series in Aplysia and Fiona; but Wierzejski ascribes a mesodermal 

 fate to the whole. 



In Limax Meissenheimer (1896) describes the cleavage of 4d to a 

 stage in which there are four cells, the anterior pair of which are the 

 smaller. In fate they serve as anlagen for mesodermal struc- 

 tures. Similar conclusions were also reached by Kofoid (1895) on 

 Limax. 



Heath (1899) has accurately traced the origin of the mesoblast in 

 Ischnochiton at the seventy-two-cell stage, and its later cleavage into 

 cells of equal size which lie bilaterally. At a more advanced stage 

 two more divisions were noted giving origin to small cells dorsally and 

 anteriorly. Heath was unable to determine whether these cells were 

 purely mesodermal or partly endodermal. 



Mead (1897) describes for the Annelid Arenicola two small cells 

 budded off from the bilaterally situated pair of mesodermal cells, and 

 by further division of the large teloblasts these cells are seen later Ijang 

 at the ends of the mesodermal bands and appear to be mesodermal in 

 fate. The same conclusions were reached regarding Clymenella, 

 though in this case the lineage has not been traced so far. In this 

 Annelid the divisions of M\ M- result in cells of nearly ecjual size, a 

 condition which may indicate a variation in later stages. 



In 1897 Wilson, having reinvestigated the history of the second 

 somatoblast of Nereis, discovered that the two small cells budded from 

 the teloblasts toward the enteron, to which in his earlier paper (1892) 



