DEVELOPMENT OF DOEIS 



935 



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

 parence of the nidamentuni 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 morula, or mulberry - 

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

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



FIG. 711. Embryonic development of Don's bilamellata : A, ovum, consist- 

 ing of enveloping membrane, a, and yolk, 1> ; 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 ; cl, foot ; g, hard plate or operculum. attached to it ; 

 Ji, stomach ; ?', intestine; in, n, masses (glandular?) at the sides of the 

 (Esophagus; o, heart (?); s, retractor muscle (?) ; f, situation of funnel; 

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



