DEVELOPMENT OF DORIS 



935 



to that of the emersion of the embryo, owing to the extreme trans- 

 parence of the nidamentum and of the egg-membranes themselves. 

 The first change which will be noticed by the ordinary observer is 

 the ' segmentation ' of the yolk-mass, which divides itself (after the 

 manner of a cell undergoing binary subdivision) into two parts, each 

 of these two into two others, and so on until a m&rula, or mulberry- 

 like mass of minute yolk-segments, is produced (fig. 711, A-F), 

 which is converted by ' invagination ' into a ' gastrula,' whose form 



FIG. 711. Embryonic development of Doris bilamellata : A, ovum, consist- 

 ing of enveloping membrane, a, and yolk, b ; B, C, D, E, F, successive 

 stages of segmentation of yolk ; G, first marking out of the shape of the 

 embryo ; H, embryo on the eighth day ; I, the same on the ninth day ; K, the 

 same on the twelfth day, seen on the left side at L ; M, still more advanced 

 embryo, seen at N as retracted within its shell ; a, position of shell-gland ; 

 c, c, ciliated lobes ; d, foot ; g, hard plate or operculum attached to it ; 

 h, stomach ; i, intestine ; m, n, masses (glandular ?) at the sides of the 

 oesophagus; o, heart (?) ; s, retractor muscle (?) ; t, situation of funnel; 

 v, membrane enveloping the body ; x, auditory vesicles ; y, mouth. 



