EAELY STAGES IN DEVELOPMENT OP BALANOGLOSSUS. 133 



yellow colour, about f mm. in length; enclosed in an elastic, 

 close-fitting, transparent eggshell (fig. 1). 



The spermatozoa dehisce in lobate sperm atophoric masses ; 

 they have spherical heads, and short flagelliform tails, with 

 which they swim actively. 



No attempt at artificial fertilisation was successful, for though 

 the spermatozoa attached themselves to the egg the result was 

 an abnormal segmentation ; the ovum dividing rapidly into a 

 large number of isolated spherules, which subsequently died. 

 It was, therefore, necessary to obtain naturally fertilised ova. 

 These were to be found without much difficulty in considerable 

 numbers deposited in the muddy sand which the adults inhabit. 



After fertilisation the appearance of the egg alters; the ovum 

 itself becomes spherical, and the eggshell increases in size, being 

 separated from its protojilasmic contents by a considerable 

 space (fig. 2). It is not at all clear by what processes this 

 change is accomplished, since the shell of the ovai'ian ovum is 

 a tough and apparently structureless membrane, which does 

 not appear to alter in character after fertilisatiou. 



The first furrow is formed in a median plane, dividing the 

 ovum into two equal parts. It passes to a considerable depth 

 (fig. 3). With regard to the subsequent segmentation I have 

 no certain observations ; for though some of the ova divided into 

 four and eight nearly equal parts, these were obtained by 

 artificial fertilisation, and the process of division was afterwards 

 continued in an entirely abnormal manner as mentioned above. 



Judging, however, from the characters of the blastosphsere 

 (figs. 4 and 18), and from the fact that yolk granules are uni- 

 formly distributed through the whole tissue, there can be little 

 doubt that the segmentation is regular and complete. 



In the next stage which was found a spherical blastosphaere 

 ■was formed (ng. 4). The walls of this were opaque, but the out- 

 lines of the cells composing them could be faintly distinguished 

 in a surface view. 



Subsequent External Changes. — Before proceeding 

 to describe the internal structure of this and the following 

 stages it will perhaps be best to describe the changes which 



10 



