THE EVIDENCE FROM DEVELOPMENT. 



175 



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, ec> 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 appear?, 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 of 

 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 FIG. 89. DEVELOPMENT OF SEA-SQUIRT. 



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 



