90 



with scattered nuclei. These two layers as at first are continued 

 into the peripheral protoplasm of the yolk. All round the margin 

 it mil be seen from the figures that certain of the cells may be 

 identified as ectodermal, that below these are cells which gradually 

 merge in the s>Ticytial entoderm. 



With the exception then of the sjTicytial change m the entoderm 

 and the similar change beginning to take place in the external layer 

 of the ectoderm, we have here a gastrula consisting of an ectoderm 

 and an entoderm united at the margm of the mdely distended 

 blastopore. The entoderm therefore is formed by delamination. 

 It is interesting to note, furthermore, that whether deliberately 

 or from the fate of position the foundations of the layer were laid 

 down before segmentation started, in the sheet of protoplasm which 

 at first was indistmguishable from the rest of the blastodisc. 



The failure to recognise in this so-called sub-germinal periblast 

 the virtual gastrula entoderm has led to most of the misunder- 

 standings with regard to Teleostean development. The proofs 

 that it is reall}^ the entoderm will now be given. 



The Postgastrula. — As soon as the changes which have been 

 described have occurred the gastrula passes into the postgastrula 

 stage, and a bilateral appearance becomes evident by the broadening 

 and deepening of the rim over an area which marks the dorsal fip 

 of the blastopore {see fig. 3, page 84). At the same time, the cslyHj 

 above referred to becomes more plain by the thinning of the roof. 

 This is brought about by the ectodermal cells of the region lengthen- 

 ing vertically and bemg drawn up between one another. The 

 anterior end of the brain becomes by the process gradual^ more 

 and more distinct (fig. 3). 



It is plain also that the anterior end of the brain originates in 

 the gastrula, and already presents an infundibular mass which 

 thins out on each side. The outer layer of ectoderm is now very 

 thm, the cells having become flattened and incorporated mth the 

 original superficial layer of protoplasm. At the margin this layer 

 of protoplasm merges into the peripheral protoplasm of the yolk, 

 where it is joined by the syncytial entoderm. The latter is now 

 entirely free from the ectodermal cells, and at the margin it is con- 

 cerned medianly in the formation of the notochord and laterally 

 with the formation of the marginal mesoderm. In both cases 



