218 LILIAN SHELDON. 



a condition would be attained similar to that found at the 

 extreme anterior end of the embryo, the now paired prolife- 

 rating masses being the prseoral lobes. Farther back the con- 

 dition would be similar to that found in fig. 15 b, in which the 

 proliferating mass has divided centrally, but the divided masses 

 remain connected above and below by a string of cells, so as to 

 enclose a secondary cavity lying on the ventral face of the 

 primary one, the two being separated only by a thin layer of 

 protoplasm. This may be seen by comparing fig. 15 b with 

 fig. 17 b ; in the latter section the proliferating mass on 

 each side has divided, part being applied to the ventro- 

 lateral corner of the ventral and part to the apposed corner of 

 the dorsal sac (fig. 17 b, Mes.). The condition found in the 

 posterior region of the last-described embryo (fig. 17 c) would 

 be attained if the proliferating mass divided, only remaining 

 connected by a layer of cells above, so that no septum would 

 be present dividing the cavity of the embryo into two. 



The main difference between this stage and the next may be 

 best seen on examination of a section through the middle 

 region of the ovum. In such a section (fig. 18 b) the two 

 cavities, which were before only separated from one another 

 by a single layer of protoplasm, are entirely distinct; each 

 being bounded on all sides by a definite protoplasmic layer, 

 and the walls, which are apposed to one another, being com- 

 pletely separated by a narrow space in which are found some 

 of the small round elements, which are present in the space 

 between the embryo and the vitelline membrane. Posteriorly 

 the two sacs communicate as before (fig. 18 c) ; anteriorly the 

 prseoral lobes are very prominent (fig. 18 a, p. o. /.), there is a 

 slight invagination of ectoderm (St.) in the middle ventral 

 line, where the mouth will be found later, and there is a pair 

 of definite hollow somites, which are the somites of the 

 prseoral lobes, lying one on each side of the yolk (fig. 18 a, S. i.) . 

 The proliferations of nuclei which constitute the mesoblast 

 are much larger and more clearly defined than in the last 

 stage (Mes.) The peripheral yolk is entirely absorbed, but 

 the whole embryo is still surrounded by the small round 



