636 



LECTURE XXIV. 



line membrane near the narrower end of the egg ; and, when the 

 folds of the vitelline membrane have been obliterated in the detached 

 egg, the germ-vesicle has disappeared or become obscured through 

 the effect of impregnation. The influence of the resulting impregnated 

 vesicle, or primary germ -cell, extends over only a small part of the 

 yolk, like the cicatricula of the bird's egg. The fission of that 

 germ-cell manifests itself by two slightly elevated prominences, as at 

 fig. 232, A, a, in each of which is a secondary nucleated cell. The next 



232 



stage shows four prominences ( ih. b) ; these by their fission form 

 eight {ih. c), of a triangular form, becoming more acute, and the 

 apex of each showing the hyaline nucleus : and now spontaneous fis- 

 sion manifests itself in another direction, and the apices of each seg- 

 ment seem to be detached, each fission-sphere {ih. d, a) having, like 

 each fission-segment (J), its proper nucleus. This figure shows 

 the sixteenfold division, affecting the central cells (a) as well as the 

 segments (&). The progressive multiplication of sperm-force-centres 

 multiplies the peripheral segments, and fills the central space with 

 smaller and more numerous derivative germ-spherules. 



The germ-mass thus formed may be divided into two layers, and 

 the outer surface becomes smooth. Its further increase by fission- 

 growth and confluence of germ-cells manifests itself by slight eleva- 

 tions from this " cambium " {fig. 233) ; the first of these is a median 

 rhomboid one, which is the basis of the 233 



trunk or mantle-sac (a); two small bean- 

 shaped prominences (Z») become the eyes ; a 

 linear elevation {d) on each side of the 

 mantle indicates the lateral halves of the 

 funnel. At this period the germ-mass is 

 two lines long and subcircular. Next 

 appear the cartilages of the funnel (c) ; 

 then the rudiments of the gills (c) ; afterwards the anterior (A) and 

 posterior {%) cephalic lobes and the arms, the latter beginning with 

 the ventral pairs (1, 2); and now there is evidence of ciliary action at 

 many parts of the embryonal surface, as, e. </., on the mantle, the 



