290 C O. AMIIT.MA.V. 



to the blastomere c, which later is veutrally placed) divides iuto 

 the primary nenroUast {x") and two mesoblasts (.r and x y) . 



5. The primary neuroblast divides into ten cells, the two 

 smaller of which \x'^) are soon broken up iuto ectodermic cells, 

 while the remaining eight neuroblasts are arranged in tico symme- 

 trical groiips at the posterior border of the blastodisc. 



6. In consequence of movements originating in the successive 

 meridional divisions of the primary neuroblast, the mesoblast (.r) 

 moves upward and backward, and so takes a position at the 

 right and a little below the left mesoblast {.ry), where it soon 

 becomes buried in the posterior ends of c and b, just beneath 

 the right group of neuroblasts. The left mesoblast [x y) lies 

 likewise just under the left group of neuroblasts. The bilateral 

 symmetry thus reached in the arrangement of the neuroblasts 

 and mesoblaats is a little imperfect in respect to the mesoblasts, 

 inasmuch as they lie nearer the left than the right side. 



7. The eight neuroblasts and two mesoblasts are the builders 

 of the germ-bands, each of which has accordingly five builders, 

 four neuroblasts, and one mesoblast. These bands appear as 

 thickened lateral margins of the blastodisc, each of which is 

 composed of four parallel lines of cells (produced by the 

 neuroblasts) and of larger subjacent mesoblastic productions. 



8. The form taken by the bands during the epibolic expan- 

 sion is determined by pressure in ttvo j^rincipal directions at right 

 angles to each other. The transverse pressure is due to the 

 expansion of the central field of the blastodisc, which is stronger 

 in a lateral direction, in consequence of the obstructions at the 

 anterior (cephalic mass) and the posterior border (neuro-blasts). 



The longitudinal pressure arises from the proliferation of the 

 neuroblasts and mesoblasts, and its direction is therefore 

 forward. The 2)redominant direction of the circumcrescent ex- 

 pansion is then in the diagonal directio7i, or obliquely forwards. 

 It is on this account that the bands which, approximately 

 sj)eaking, are fixed against the cephalic mass, exhibit the 

 strongest curve in their anterior halves. 



9. A solid invagination of the ventral [c] and lateral blasto- 

 raeres {a and b) accompanies the epibolic extension of the 

 blastodisc, and is caused by this extension (reckoning from the 

 moment the primary neuroblast begins to divide) . This invagi- 

 nation consists mainly in forcing c towards tlie dorsal side, in 

 such a manner that the broad ventral side becomes narrow and 

 the narrow dorsal side becomes broad. The original wedge- 

 shape is thus inverted. 



10. About the time the germ-bands begin to form, the nuclei 

 of a, b, and c, abandon their central position and pass to the 

 ])criphery, where they multi))ly by free division during the 

 whole i)eriod of epibolic invagination. The enioplasts thus 



