No. 2.] THE MESODERM AND MESENCHYM. B57. 
cells, the mesenchym tissue being here unipolar and bicellular, 
instead of multipolar and multicellular in origin. 
Further, in meroblastic ova, from which the gastrulae are 
not of the invagination type, but sterroglastulae, etc. (as in the 
Cephalopoda, Sauropsida, Hirudinea, Turbellaria, most Arthro- 
poda, etc.), the whole process of production is so much modified 
by the amount of yolk in certain of the blastomeres, that the 
mode of development of the mesoderm can with difficulty be 
referred to the three more primitive modes of development 
given above. 
The examples given show how multifarious the development 
of the mesoderm is in different animal groups. 
Again, there is no sharp distinction between mesenchym and 
mesoderm, but only a difference of degree; as is shown in a 
large number of the newer embryological researches. Thus, 
while the term “mesenchym”’ is generally applied to cells of 
the third primitive embryonic layer which are not united 
together in a continuous mass, and while “mesoderm”’ is 
applied to cells of that layer when from the beginning they 
are in contact with one another, forming a mass of cells (‘‘mes- 
oderm-stripe’’), we find, in the case of the production of the 
mesoderm through typical pole-cells, that the two cells may be 
or may not be in contact, but that the mesoderm cells prolifer- 
ated by each construct a solid cell mass. In fact, whether the 
cells of the third embryonic layer are not united (‘“‘mesenchym’’) 
or are united (‘‘mesoderm,” sensu strictiort) would seem to 
depend upon two factors: (1) the comparative size of the arch- 
icoel and (2) upon the modes of production of the third embry- 
onic layer, which modes we have found to intergrade. And in 
such groups as the Mollusca and Arthropoda, it is difficult to 
decide whether the third embryonic layer is to be classed as a 
“mesoderm” or as a “mesenchym.” Accordingly, these two 
terms express but extremes of a series, and morphologically 
cannot be sharply distinguished. 
Lastly, as to the morphological value of the cavity enclosed 
by the mesoderm cells. This cavity is a derivative of the arch- 
icoel in all those forms where the mesoderm is formed (1) by 
multipolar, mesenchymatic delamination, or (2) by pole-cells, 
