G5 



features : — 1st The tloiible tube, neural and visceral ; 2ndlv, 

 the formation of the neural tube by means of medullary folds ; 

 Srdly, the notochord. 



While these changes have been going on in the material 

 derived directly from the cleavage masses of the vitellus, the 

 yellow vesicles fovuid in the gelatinous envelope have also 

 suffered transformation. . Losing their colour they acquire a 

 nucleus, their substance becomes distinctly protoplasmic, 

 and they send out processes in the gelatinous substance ; in 

 ffict they take their place as constituent cells of the mantle. 



Very soon after the larva has left the egg it seeks to fasten 

 itself to some object. This it accomplishes by means of the 

 three processes above described. Forthwith the tail is re- 

 tracted, the homogenous, highly refractive, core becoming 

 broken up, and the elongated cells of the sheath being drawn 

 together and appearing as conspicuous rounded cells. The 

 muscular coat at the same time is resolved into round cells 

 which lie in the cavity of the body. The gelatinous envelope 

 of the tail is also retracted and very much wrinkled. A little 

 later on the remnant of the tail appears as a confused heap 

 of fatty tissue in Avhich veritable cells can hardly be distin- 

 guished, and which is gj-adually absorbed as Krohn describes. 



The epidermic processes by which the larva attaches itself 

 in great measure disappear, the constituent cells "wander- 

 ing^' into the gelatinous sheath, and the portion of the body 

 which serves for attachment widening out into a foot. 



The neural vesicle collapses, the cavity becoming less, and 

 the cells constituting the ganglion becoming round, small, 

 and more alike. Later on the organ is reduced to a heap of 

 cells gathered irregularly, without distinct outline, round the 

 pigment spots, and thus dwindles into the permanent gang- 

 lion of the adult animal. Very many of the cells originally 

 forming part of the nervous system are transformed into 

 blood-corpuscles. 



The fore part of the alimentary canal, by appropriation of 

 the cells lying round it, acqidres thickened walls, and is 

 dilated into a wide branchial sac which is marked off by a 

 constriction from the rest of the digestive tube, the latter also 

 growing stouter and its convolutions becoming more obvious. 



At the front part of the branchial sac the wall of the sac 

 and the wall of the body (i.e. the mucous and epidemic 

 layers) coalesce in two oblong patches on either side. The 

 centre of each patch is absorbed, and the holes thus formed 

 widen transversely into the branchial slits. The succeeding 

 slits are formed in a similar way. 



Close to fore and under corner of the branchial sac there 



VOL. X. — NEW SEE. E 



