i'llELlMINARY NOTE ON THE MESODERM FOKMATION OE PULMONATA. 'ji 



from the fifth. The luombers of the sixth and the seventh generation of 

 micronieres are remarkable for tlieir small size and their comparatively 

 large, highly refractive nucleus. Up to this stage, the micromeres, four 

 in number to each generation, are radially arranged. But interruptions 

 to this arrangement soon begin to set in, leading to the establishment of 

 a bilateral symmetry and henceforth the order of the succession of 

 events shows less regularity than heretofore. The mesoderm now 

 undergoes an unequal division, of the outcome of which, the smaller, 

 that adjoins the cross-furrow on the vegetative pole^ is the entoderm, 

 while the larger, is the TJrmesoderm of German authors (tig. 4). Usually 

 at about this time, a peculiar and important phenomenon occurs. 



Namely, the two micro- 



meres, belonging to the 

 fourth generation and ly- 

 ing on both sides of the 

 TJrmesoderm, divide or 

 rather bud out a cell near- 

 ly as small as the micro- 

 mere of the sixth and the 

 seventh generation and 

 with a similarly refractive 

 nucleus. The bud on the 

 left side of the Urmeso- 

 derm is directed anterior- 

 \y, that on the right side 

 toward the median line 



/ 



(lilnil 



/ 1 



Smc: 



V/> 



'//MC 



T 



5uuc,\ 



€ftt. 



q . 



7mic\ 



ni^n. 



cnt. 



Os. 



imc. 



mes. 



Diagram showiuy tho order ut' foruiatioii of dit- 



rent cleavage-segments, 

 blm.— Bliistomoro of the 1-cell stage, 

 mac— Macrouieres, 1 iu number, one of which is the 



eutouieeoderm and the rest, entodermicmacromeres. (tiS- ^i ^ -^v 

 1-S mic — lst-8th generation of micremeres. 

 cut. 1 — Entderm cell that arises from the eutomeso- 



drem cell, 

 ent. 2 — Entoderm cells, three in number, tliat arise 



from the entoderujic macrouieres. 

 urm. — Urmesoderm cells, two in number, 

 mes. — mesoderm cells. 

 X. — cells, two iu number, arising from two micromeres 



of the ith generation. 



From this stage, all 

 the micromeres begin to 

 change their configura- 

 tion, gradually becoming 

 more flattened. This is 

 especially the case with 



