242 LILIAN SHELDON. 



nuclei (Mes.) are present at the ventro-lateral corners of the 

 primary sac, aud there are also iudications of a proliferation of 

 the nuclei at those corners of the secondary sac which are 

 apposed to the thickenings on the primary one. 



In a section through the posterior region of the egg (such 

 as is represented in fig. 17c), the septum dividing the two sacs 

 from one another has disappeared, so that they are in free 

 communication with each other. Thus the central cavity of 

 the embryo is continuous from the anterior extremity of the 

 embryo round the posterior end of the egg to the tip, which 

 was found lying on the ventral face of the head of the embryo 

 between the prseoral lobes (vide fig. 17 a, post. Em.) The 

 longitudinal thickenings along the ventro-lateral borders of 

 the sacs coalesce shortly behind the point where the septum 

 disappears; that is, the thickening on the right of the primary 

 sac coalesces with that on the right of the secondary one, and 

 similarly those on the left ; so that on each side of the embryo 

 there is a thickened ridge which starts just behind the prseoral 

 lobes, and is continued round behind the septum along the 

 sides of the ventral sac. These thickenings are the mesoderm. 

 The endoderm is only represented by a few scattered cells in 

 the yolk which fills the embryo. The embryo, therefore, con- 

 sists of a sac which, except at the posterior end of the egg, is 

 divided into a dorsally- and a ventrally-lying one by a longitu- 

 dinal horizontal septum. The whole embryo is filled with yolk, 

 and is surrounded by a thick layer of the small round bodies, 

 outside which is the vitelline membrane. 



Several intermediate stages are obviously wanting between 

 this embryo and the previous one, and it is therefore not possible 

 to state positively how the one developes into the other. It 

 seems possible, however, that the cavity (figs. 15 b, p) which 

 was present in the proliferating mass of nuclei in the previous 

 stage, corresponds to that which constitutes the ventral sac in 

 the anterior portion of the egg last described (fig. 17 a, post. 

 Em.). If the proliferating mass were anteriorly to divide 

 completely, only remaining attached by a string of cells on the 

 ventral surface of the embryo, constituting the ventral ectoderm, 



