1904.] NATURAL SCIENCES OF PHILADELPHIA. 343 
anteriorly which lie 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 inn 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 Limazx 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 
Limaz. 
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 lying 
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 equal 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) 
