Primitive Cell-layers of the Embryo. 329 



Tlie observations of Miklucho-Macleay*, which have been 

 followed up in a masterly way by his teacher Professor 

 Hackel of Jena, first demonstrated the relationship of Sponges 

 and Coelenterata. The Planida-tmhYyo of a calcareous sponge 

 ( Guancha bianco) is made known in Macleay's paper ; 

 O. Schmidt has figured that of another {Dunstervillia). The 

 embryo of Spongilla, as described by Lieberkiihn, is also a 

 Planula. 



The retention of the Diploblastic constitution throughout life 

 by the Coelenterata serves as an important fact in determining 

 the homogenies of the perigastric and canal systems of the 

 corals and medusoids. It is clear enough that they are merely 

 diverticula, or portions of the primitive gastric cavity. As 

 such they can have no homogenetic, but merely a homoplastic, 

 agreement with the vascular and perivisceral systems of higher 

 animals, the origin of which will be pointed out below. The 

 fluid which they contain will also be seen to be of a different 

 nature from chyle or blood, and, in fact, is merely a diluted chyme. 

 In the histological differentiation of Coelenterata the outer 

 layer of cells gives rise to muscular fibre, and also represents 

 a nervous system ; in the case of Hydra the fibres are con- 

 tinuous with the large ectodermal cells (Kleinenberg) , whilst 

 in others {Medusce &c.) deep-lying cellular elements of the 

 nature of muscular and connective tissue develop from the 

 ectoderm. The endodermal cells are confined to vegetative 

 functions. The origin of generative products will be discussed 

 below. 



C. Development having proceeded, as in the Diploblastica, 

 to the production of an ecto- and endoderm, or an epi- and 

 hypoblast, with primitive gastric cavity bounded by the latter, 

 a third layer of cells makes its appearance between these two, 

 whence taking its precise origin is notyet determined. A portion 

 of this middle layer becomes more especially adherent to the 

 ectoderm, another portion more especially to the endoderm. 

 The separation between these two portions of the new mid layer 

 may be complete so as to leave a wide cavity, or it may never 

 be carried to any extent ; but whatever extensive cavity or 

 partial channels make their appearance, or whatever mesh- 

 bearing or sponge-like character the mesoblast takes on, so as 

 to produce an imperfect continuity between its more super- 

 ficial and deeper parts, connected and bound together, it may 

 be, by branched cells — such cavity, channels, or spongy tissue 

 ai-e more or less complete representatives of the blood-lymph 

 system. The organisms characterized by the presence of these 

 * Jenabche Zoitschrift, 1868, p. 221, 



