KECENT LITEEATUEE OF BLASTODEEM. 27 



of this with ectoderm at the margin of the blastopore, and in the higher forms, especially 

 mammals, may even be largely derived from ectoderm. It is nevertheless for many reasons 

 probable that the origin in a pair of hollow diverticula, as above described, is to be looked 

 upon as the typical one. and that as a solid outgrowth, subsequently becoming split or hollow, 

 as a secondary modification. 1 It is questionable, however, whether there is so considerable a 

 difference between the external and internal portions of the wall of the diverticulum that 



ccl. 



Fig. 29. SECTIONS ACROSS AN AMPHIOXUS EMBRYO OF ABOUT THE STAGES SHOWN IN FIO. 28, I. TO 



III. (Hatschek.) 



n.g., neural groove ; n.c., neural canal ; cA, rudiment of notochord ; mes. som., mesoblastic somite. 

 In I. , its cavity is in free communication with the alimentary cavity ; ep, epiblast ; hy, hypoblast ; al, 

 alimentary cavity. In III. the cavity of the somite has extended on either side of the alimentary 

 canal and forms a ccelom, or body cavity (cce). 



these two plates of mesoblast should be regarded each one as of equal morphological importance 

 with the epi- and hypoblast. 



It may also be doubted whether the parablastic cr mesenchyme elements are essentially 

 different in their origin from the rest of the mesoblast. 2 In forms which are regarded as 

 most typical, such as Sagitta and Amphioxus, they are not distinct in origin from that layer. 

 In the simpler forms amongst the Craniata, as Cyclostomata and Amphibia, no origin 

 distinct from the rest of the mesoblast has been described for these elements, nor has it been 

 seen in mammals, in which, indeed, it is difficult to conceive an independent source for them. 

 It is only in the highly modified meroblastic ova that appearances have been noted which have 

 seemed to justify the ascribing a peripheral origin to the parablastic elements. But the evidence 

 which has been hitherto adduced in favour of this view cannot be regarded as sufficient to 

 justify its unconditional adoption, and it must be regarded as equally open to consideration 

 whether the derivation from that part of the blastoderm which is most closely connected 

 with the source of nutriment, viz., the yolk, of those elements which are to form the blood 

 and blood-vessels, and otherwise to minister to the nutrition of the early embryo is not to be 

 explained by the modified physiological conditions of these telolecithal ova. 



EECENT LITERATURE. 



Balfour, F. M., On the structure and homologies of the germinal layers of the embryo. Quart. 

 Journ. Microsc. Sc., vol. xx., 1880. 



Balfour, P. M. and P. Deig-hton, A reneiced study of the germinal layers of the chick. 

 Quart. Journ. Microsc. Scienc., vol. xxii., 1882. 



Beneden, Ed. van, Recherches sur I' 'embryologie des mammiferes. La formation des feuilleis 

 chez le lapin. Archiv. de biologie, t. L, 1880 ; Sur Devolution de la ligne primitive, la formation 

 de la notocorde et du canal cordal chez les mammiferes (lapin et murin). Bulletin de 1'academie 

 royale de Belgique. Ann. v., sdr. iii., t. xii., 1886. 



Beneden, Ed. van u.nd Ch. Julin, Observations sur la maturation, let fe'condation et la 

 segmentation de Veen f chez les cheiropteres. Arch, de biol., t. i., 1880. 



Bonnet, R., Ueber den Primitivslreifen und die Chorda der Wiederkauer. Sitzungsber. d. 

 Gesellschaft f. Morphologic u. Physiologic zu Munchen, 1886 ; Bcitrage zur Embryologie der Wiedfrkduer. 

 Arch, ftir Anat., 1884, 1889. 



1 R. and 0. Hertwig, "Die Ccelomtheorie," Jena, 1881. 



2 Cf. Balfour, "Comparative Embryology," vol. ii. pp. 296, 297. 



