TUE MOST IMPORTANT PROCESSES IX GASTRULATIOX. 28 1 



with a single axis, it assumes the bilateral vertebrate form. 

 3. The body-cavity (coeloma) arises, in consequence of the 

 disconnection of the skin-fibrous layer and the intestinal- 

 fibrous layer. 4. Along the central line of the dorsal 

 surface the nerve-centre appears in the form of a trench- 

 shaped furrow; it then changes into the spinal-tube and 

 completely detaches itself from the skin-sensory layer. 

 5. Immediately below the spinal tube, the notochord origi- 

 nates from the central part of the skin-fibrous layer, while 

 the side parts of the same layer form the true skin and the 

 trunk-muscles ; the latter articulate themselves into meta- 

 mera. 6. In the outer stratum of the intestinal wall, in 

 the intestinal-fibrous layer, originate the main blood-vessels, 

 a dorsal vessel (aorta) above the intestinal tube, and a ven- 

 tral vessel (primitive vein) below the latter. 7. The intes- 

 tinal tube separates into two main parts ; the gill-intestine 

 in front, the stomach-intestine behind. Several gill-open- 

 ings form on either side of the gill-intestine. 8. The 

 intestinal tube acquires two new openings, a mouth in front, 

 an anus behind ; the original primitive mouth of the Gas- 

 trula closes. 9. Close by the intestine and notochord, and 

 on either side of them, arises a tube which separates urine, 

 and which opens into the body-cavity in front, outside the 

 body in the rear ; this is the primitive kidney canal. 

 10. Close by this canal, between it and the notochord, develop 

 the rudiments of the sexual glands (the testes and ovary), 

 in the form of roundish cellular masses, which penetrate 

 from the wall of the body-cavity to this position (the un- 

 defined boundary of the skin-fibrous layer and the intestinal- 

 fibrous layer). 90 



These chief, fundamental, and palingenetic acts in the 



