[120] The above list is somewhat tentative; and future investigations will probably shew that many of the statements at present current about the position of the blastopore are inaccurate.
[121] The forms in which the position of the blastopore in relation to the mouth or anus is not known are marked with an asterisk.
[122] J. Parker, “On the Histology of Hydra fusca,” Quart. Journ. Micr. Science, vol. XX. 1880; and El. Metschnikoff, “Ueb. die intracelluläre Verdauung bei Cœlenteraten,” Zoologischer Anzeiger, No. 56, vol. III. 1880 and Lankester, “On the intracellular digestion and endoderm of Limnocodium,” Quart. Journ. Micr. Science, vol. XXI. 1881.
[123] Vol. II. p. 149.
[124] The Hertwigs (No. [270]) have for instance shewn that nervous structures are developed in the hypoblast in the Actinozoa and other Cœlenterata.
[125] There is considerable confusion in the use of the names for the embryonic layers. In some cases various tissues formed by differentiations of the primary layers have been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out the inconvenience of this nomenclature. In the case of the Cœlenterata it is difficult to decide in certain instances (e.g. Sympodium) whether the cells which give rise to a particular tissue of the adult are to be regarded as forming a mesoblast, i.e. a middle undifferentiated layer of cells, or whether they arise as already histologically differentiated elements from one of the primary layers. The attempt to distinguish by a special nomenclature the epiblast and hypoblast after and before the separation of the mesoblast, which has been made by Allen Thomson (No. [1]), appears incapable of being consistently applied, though it is convenient to distinguish a primary and a secondary hypoblast. A proposal of the Hertwigs to adopt special names for the outer and inner limiting membranes of the adult, and for the interposed mass of organs, appears to me unnecessary.
[126] The causes which give rise to a retardation of histological differentiation will be dealt with in the second part of this chapter which deals with larval characters and larval forms.
[127] The connective-tissue test of the Tunicata, though derived from the epiblast, is not really an example of such a differentiation.
[128] M. L. Ranvier. “Sur la structure des glandes sudoripares.” Comptes Rendus, Dec. 29, 1879.
[129] A. Götte, “Vergleich. Entwick. d. Comatula mediterranea.” Archiv f. mikr. Anat. vol. XII. p. 597.