344 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Vertebrata the lateral mesoblastic masses begin to be developed before 

 the gastrula-invagination is completed, and here it seems as if the 

 mesoblast arose from the ectoblast. However, in both cases the final 

 result is the same, and we cannot justly speak of any real difference 

 in the genesis of the middle layer. 



3. The explanation of the diflerence of opinion as to the origin of 

 the notochord is partly to be found in the fact that the observers of 

 some stages have examined embryos at a time when the median band 

 passes directly on either side into the mesoblast ; and they, therefore, 

 have ascribed a mesoblastic origin to the chord. Others have studied 

 development more particularly at the period when the chorda-endo- 

 blast has become separated from the mesoblast and lies above the 

 enteric glandular layer in the form of a thickened band of cells ; and 

 they have asserted the hypoblastic origin of the chord. 



The following considerations appear to have an important bearing 

 on the questions at issue. (1) Before the notochord becomes de- 

 veloped the embryo is in part bilaminate, consisting of an ectoblast 

 (medullary plate), and of chorda-endoblast which takes part in the 

 delimitation of the enteric cavity. (2) On either side of this portion 

 (median band) the embryo is trilaminate, if we regard the mesoblast 

 as a simple layer, and quadrilaminate if we regard the mesoblast 

 as consisting of parietal and visceral cell-laj^ers, which only become 

 distinctly separated on the appearance of the coelom. (3) In no 

 Vertebrate does the mesoblast arise by cleavage. (4) The mesoblast 

 is only connected with the bounding cell-layers at the blastopore or 

 at the primitive groove, where all three germinal layers are connected 

 together, and at the two sides of the chorda-endoblast. (5) The 

 mesoblast arises peripherally and extends forwards, backwards, and 

 ventrally ; in front of the blastopore it forms a paired rudiment 

 separated by the chorda-endoblast, while behind the blastopore it is 

 unpaired. (6) As the chief portion of the material for the growth of 

 the mesoblast is derived from the cells, which, at the blastopore, or at 

 the primitive groove, pass from without inwards, we are entitled to 

 say that the process of invagination which commences with the forma- 

 tion of the gastrula is carried on into later stages of development. (7) 

 The process by which the paired mesoblastic bands separate from the 

 neighbouring cell-layers and undergrow the notochord to inclose the 

 enteron, may be so far modified that a lamella of cells from the 

 chorda-endoblast may take part in it (Anui'a). 



The author supports the important doctrine that the ingrowth of the 

 mesoblast may be looked upon as an invaginative process of epithelial 

 lamellae by the following facts : (a) The mesoblast arises as a con- 

 nected mass from masses of epithelial lamellae. (/8) In all Vertebrates 

 a cleft appears early in the mesoblast which is bounded by epithelial 

 cells, of a cylindrical or cubical form. The parietal and visceral 

 mesoblast are, as is well seen in Elasmobranchs, at a very early stage 

 epithelial lamellae, (y) From these there arise true epithelial mem- 

 branes and glands. (o) This view is supported by what is seen in 

 Amphioxus. (c) The objection that the mesoblast of the Vertebrata 

 arises as a single cell-mass and cannot therefore be regarded as equi- 



