224 



ZOOLOGY. 



has to feed for itself on the floating diatoms and the like 

 in the sea- water. This escape from the egg-membrane 

 occurs about eight hours after the beginning of segmenta- 

 tion. 



Ciliated larvae are common in almost all groups of marine 

 Invertebrata ; but the Amphioxus is the only Vertebrate 



that passes through this stage of free- 



swimming by means of cilia. 



10. Notochord and Mesoblast. The next 

 changes must be briefly described. They are 

 indicated in figs. 113, B and c, which show 

 a series of transverse sections at different 

 stages. The ordinary epiblast at the sides 

 of the neural plate begins to grow in towards 

 the middle line so as to cover in the neural 

 plate. Posteriorly these neural folds close 

 in over the blastopore, forming there a 

 tunnel called the neurenteric canal, which 

 continues the archenteron dorsalwards and 

 forwards to over the surface of the neural 

 SECTION" plate. Meanwhile the median dorsal portion 

 THROUGH A LARVA o f fc ne primitive hypoblast becomes thickened 



OF AMPHIOXUS. . . r _. ., j <? .1 .LI i 



T ^ -1^1 *i, longitudinally, and from this thickening a 



In the middle is the & J . . 



notochord, with longitudinal rod of cells is pinched oil : this 



pSsTnttheris the future notochord. Right and left of 



side of it. (From the notochord a metameric series of hollow 



Hatechek.)* 'outgrowths of the archenteron appear, and 



push their way in between epiblast and 



hypoblast, as they develop. These are the myocoelomic 



pouches, the cells of whose walls constitute the mesoblast 



(fig. 114). 



11. The Three Germinal Layers, The remainder of 

 the primitive hypoblast, after the mesoblast has been 

 separated from it, is called simply hypoblast, and the cavity 

 it encloses (which is the archenteron minus the myocoalomic 

 cavities) is called the mesenteron. Epiblast, mesoblast, and 

 hypoblast are known as the three germinal layers. At this 

 stage, evidently, they are all simple epithelia : it is riot till 



" 



