140 



s. HÂTTA. 



florsnl limh or tlio part that originally formed the wall of the giit- 

 diverticiiluin ber-omes iiicorporatcd in the mesoblastic mass; the ventral 

 half which was originally turned towards the definitive gnt-cavity, 

 loosens itself from the dorsal half and passes t(^v>"ards the median line 

 beneath the chorda and hypochordal cells, nntil it meets and fuses 

 Avith its fellow of the opposite side to form tlie definitive roof of the 

 enteric canity. 



'i'lie ]^rocess of mesol^last formation in the cephalo-cervical region 

 as given above, is exactly like what we know to go on in Am.phioxu<^. 

 Call)erla, Scott, and Shiple)' do not make any mention of such a 

 primitive mode of the mesoblast formation in Pctromijzon. Ivnpffer's 

 descriprion of the process in the head region accords well witli mv 

 own o1)servations, liowever with tliis great difference that according 

 to him tlie chnrda is cnt off and completed long before the mesoblast 

 is f rrmod. That such is not the case, T \\[\\e no doubt in my own 

 mind; my sections speak too plainl}^ for any doul)t. ^ly account also 

 agrees exactly with tlie results recently inade known by Mitsukuri^ in 

 Cliel<mia. If one compares my fig. 19 with his fig. 11, one will be 

 easily corivinced that a certain stretch of epithelium on each side of 

 the median c\\iW(\'i\' anläge becomes eventually incorporated in the meso- 

 blasf. 



Let us now pass to the truidv and observe the mode of mesoblast 

 formation in tliat region. Fig. 13 shows a transverse section from 

 tlie d(>rsal region of an endjryo in which no cephalic protuberance is 

 seen as yet. In the median line the epiblast shows a thickened 

 medidlai'v plate {n. ]).) formed of high columnar cells. Immediatelv 

 beneath, l)ut di.itinctl)^ separate from, tliis plate, there is the chorda-hypo- 

 blast (ch. //.) forming tlie entire roof of the invaginated tube. Its 



1. Fui'ther studies on the formation of the Germinal Layers in ('helonia: The Journal of 

 the College of Science, Imp. Univ., Japan. Vol. V. p. T. 



