No. I.] THE EMBRYOLOGY OF CREPIDULA. 69 



3. The Primary Mesoblasts. Figs. 32-^g. 



The two anterior cells resulting from the preceding division are 

 still mesentoblasts, and the mesoblastic and entoblastic sub- 

 stances in these cells are not completely separated until after 

 two more purely bilateral divisions. The first of these divisions, 

 Fig. 32, occurs immediately after the formation of the primary 

 enteroblasts, and gives rise to two small cells, m^ and m'^, Figs. 

 33, 35, 36, which are X}ci& pri^nary mesoblasts, i.e., they are the 

 first purely mesoblastic cells formed. They are not, however, 

 strange as it may seem, the "pole cells" of the mesoblast ; 

 they or their derivatives form the anterior or distal end of the 

 mesoblastic bands, and not the posterior, growing end. The 

 tv/o posterior products of this division, Mie' and M^e^, still 

 contain both mesoblast and entoblast. Finally, by another 

 division of these two cells, Fig. 41, the mesoblast and entoblast 

 are completely separated. This division is also purely bilateral, 

 and results in the formation of the mesoblastic te lob lasts, M^ 

 and M2, Figs. 42 et seq., and the secondary enteroblasts, e^ and 

 e2. The latter cells lie in front of and overlap the primary 

 enteroblasts, E^ and E^, and like these they are in contact 

 with each other along the mid line. The mesoblastic teloblasts 

 lie laterally to the secondary enteroblasts, and are far removed 

 from each other, Figs. 42 et seq. Counting from the formation 

 of the mesentoblast, 4d (the primary mesoblast of most authors), 

 it has taken eight cell divisions to bring about the complete 

 segregation of the mesoblast and entoblast in this region of the 

 ^ZZ- This can be seen at a glance in the table of the lineage 

 of 4d on the following page. 



It is at once apparent from this table that there is a very 

 intimate connection, at least in origin, between the mesoblast 

 and the entoblast. The cell 4d seems to contain all the 

 mesoblastic substance which was originally present in the 

 macromere D ; but it also contains a considerable amount of 

 entoblastic substance, less than half the cell being destined to 

 form mesoblast. The separation of the mesoblastic and ento- 

 blastic substances in this cell begins by the formation of the 

 primary enteroblasts, E^ and E^, on the posterior side of the 



