THE EVIDENCE FROM DEVELOPMENT. 



'75 



included, in possessing a germ-vesicle (a) and germ-spot (b\ Once 

 again, as in the sponge, we meet with the process of egg- segmentation 

 (2), resulting in the production of a morula (3). Then the cells of the 

 morula arrange themselves to form the two layers (Fig. 89, 4, <?<:, en) 

 as in the sponge, the outer layer being pushed inwards upon itself so 

 as to form a central cavity (d\ much as a night-cap is so modelled to 

 fit the head. Thus our " gastrula-stage " (4) once again appears, and 

 in the life-history of an animal very far removed from the sponge in 

 structure and relationship. 



From this stage, common alike to sea-squirts and sponges, 

 ascidian development begins to specialise itself. Another opening 

 or depression (b] appears above the opening which formerly led into 

 the gastrula-body. Within 

 this depression, which at first 

 communicates by an opening 

 (o) with the exterior, a part 0* 

 the outer layer is contained, 

 and finally becomes shut off 

 from the other portions of 

 that layer. This separated 

 and confined part (b} of the 

 outer layer becomes the 

 nervous system of the sea- 

 squirt. Next in order, we 

 find the body to extend itself 

 behind, so as to form a well- 

 marked "tail" (Fig. 89, 5), 

 within which a rod-like body, 

 the urochord (n), is formed. 

 Overlying this body at its 

 front portion, the nervous 

 system (/) just mentioned is 

 further elaborated ; and mus- 

 cular elements become de- 

 veloped in connection with 

 the tail and its contained rod. 

 Meanwhile the beginnings of a digestive system (d] and of the breath. 

 ing- sac (g) are being formed, and at this stage the young sea- squirt 

 appears to be actively mobile, and to swim freely in its tadpole-like 

 stage of development. Fixing itself thereafter by specially developed 

 points of attachment, there begins a process of apparent degeneration 

 in our as yet undeveloped ascidian. The tail wholly disappears, and 

 the nervous system degenerates until but a mere fragment remains ; 

 and with an alteration of the form of the body, and some modifica- 

 tion and further development of the other systems of organs (such as 



FIG. 89. DEVELOPMENT OF SEA-SQUIRT. 



