EMBRYOLOGY OF CLEPSINE. 269 



blastomere (c) is a linear depression which widens a little 

 towards c. This shallow depression, the anterior end (m) of 

 which mariis the place of the future pharyngeal orifice (mouth), 

 is destined to be continuous with the primitive groove formed by 

 the conjunction of the two germ-bands. 



/. Movements among the Cleavage-products. — In passing from 

 stage 33 to stage 38 important changes of position take place 

 on the lower pole of the Blastula (this term will apply to stages 

 23 — 28), which have hitherto escaped observation. These 

 changes originate in the cleavage of x^ (fig. 30). As this cell 

 begins to divide it lengthens transversely and thus disturbs the 

 equihbrium of pressure. The pressure on the lateral walls of 

 the Blastula (figs. 29 and 30) is increased, while the pressure 

 on X from above and behind is correspondingly diminished. 

 The consequence is that the mesoblast (.r) moves backwards and 

 upwards, followed by (c) and, to a certain extent by a and 6. 

 The next division of the neuroblasts (fig. 33, x^) operates in 

 the same way, except that the pressure is exerted still further 

 above the equatorial plane of the Blastula. This gradual eleva- 

 tion of the plane of pressure is still more evident in stage 35 

 and 36, and must obviously continue as long as the neuroblasts 

 go on dividing in perpendicular planes. The lateral blasto- 

 meres (a and d) are thus ]mshed not only towards the ventral 

 blastomere (c) but also downward towards the mesoblast (x). 

 As X moves slowly upward and backward its lower face becomes 

 more and more covered by a, b, and c. 



Fig. 36 shows how far x has travelled from its original 

 central position. It now lies at the right oi x ?/ and a little 

 below it. In the course of these movements, none of which are 

 active migratory motions, the left mesoblast {x y) becomes quite 

 buried in the left blastomere {a) , but does not usually disappear. 

 The right mesoblast {x) however, is soon completely enveloped 

 by what we may now call the posterior ends of a, b, and c (fig. 

 37). The relative positions of the two mesoblasts at this time 

 (:^8 h. 30 miu.) are seen in fig. 81 (transverse section just 

 behind the blastodisc). The right mesoblast lies under the 

 neuroblasts in the ventral blastomere {c) , a little to the right of 

 the middle of the Blastula and on a somewhat lower plane than 

 the left mesoblast [x i/). In the upper pole of each mesoblast 

 fine granular protoplasm is collected around the nucleus. Above 

 each mesoblast — more distinctly over the left — is seen a line of 

 small cells composed of the same kind of protoplasm. That 

 these cells arise from the upper (formative) poles of the meso- 

 blasts is established by serial sections, by dissection, and by 

 examination of the living egg. If eggs at this epoch are heated 



