268 C. O. WHITMAN. 



blastomeres [a, h, and c). Fifteen minutes later the neuroblasts 

 {x^) begin to divide, the cleavage progressing from above down- 

 wards and cutting each into two equal parts (fig. 84, or', 24 h. 3U 

 min.) . Two hours after the formation of these four neuroblasts 

 (j-g), the two inner ones begin to push each other apart by means 

 of nipple-like protrusions of their contiguous faces (fig. 35, x'^, 

 26 h. 30 min.). This pushing forces at the same time the two 

 outer neuroblasts somewhat farther apart. In the course of ten 

 minutes the two protrusions are completely constricted from the 

 mother-cells. The two small central cells (j'^} thus formed are 

 always very distinct for a short period, but soon break up into 

 four small ectoderm cells (fig. 37). One hour after the pro- 

 duction of these two cells {x^) the mother-cells divide again, 

 producing this time two cells from their anterior faces (fig. 37, 

 x^). The last two cells are generally more or less covered with 

 the small cells of the blastodisc and therefore difficult to 

 recognise. A little later the two outer anterior neuroblasts (.r^) 

 divide, producing the two cells x^ which lie between x^ and x^ of 

 each side, and which appear at first considerably smaller than 

 the other neuroblasts. There are now eight neuroblasts (fig. 

 37, 28 h. 30 min.) arranged in two symmetrical groups at the 

 posterior border of the germinal disc. The role played by tliese 

 neuroblasts will become apparent when we come to speak of the 

 formation of the germ-bands. 



e. The four primary Ectoblasts and their relation to the Mouth. 

 These ectoblasts dill'er Irom the blastomeres only in having a 

 little more protoplasm in proportion to the amount of deuto- 

 plasm. A transverse section of stage 26 (fig. 79, S) shows 

 that they are not enuclear buds as supposed by Robin. Fig. 

 79, T.) shows a nucleus of one of the ectoblasts as found in 

 stage 23. This section (S) cuts two of the ectoblasts, between 

 •which and the lateral blastomeres {a and U) two smaller cells of 

 similar composition are wedged. The nuclei of the blastomeres 

 (one is seen in a), lie near the upper surface at this time, which 

 is in harmony with the production of small ectodermic cells from 

 their upper faces — a ))rocess which continues at least as late as 

 stage 34. After maintaining their individuality for a comj)ara- 

 tively long lime, the four ectoblasts are found at length (fig. 

 34, 24 h. 30 min.) to have undergone two or more divisions ; 

 but the division-])roducts remain in situ, so that one can easily 

 recognize the limits of the original cells. The same is still true 

 in stage 37. lii stage 38 it is jjlainly to be seen that the daughter- 

 cells of the ectoblasts form the ce])halic portion of the embryo. 

 They rise slightly above the niveau of the neighbouring cells. 

 Jiei/innnig in their centre (///) and running towards the ventral 



