538 SUMMARY OF CURRENT RESEARCHES RELATING TO 



we must, however, note that the author speaks of " les obscurites et 

 les indecisions des ideas de Balfour," and cannot refrain from sug- 

 gesting that it is possible that the English naturalist has suffered in 

 translation. In many points, Balfour's article, published in 1878, is 

 in complete agreement with that now before us. 



Sabatier tells us that his essay is to be looked upon as offering a 

 rational explanation, which may be acceptable for the present, and 

 promises a future essay on the relations of heredity to the sexual 

 polarization of the elements. 



Embryonic Germinal Layers and the Tissues.* — A. KoUiker 

 states the conclusions of a valuable descriptive and critical essay in the 

 following terms : — 



1. In all multicellular organisms all the elements and tissues arise 

 directly from the fertilized egg-cell and the first embryonic nucleus ; 

 and there is no such difference as is expressed by the terms archiblast 

 and parablast. 



2. The tissues first differentiated have the characters of epithelial 

 tissues, and form the ectoblasts and endoblasts. 



3. All the other tissues arise from these two cell-layers ; they are 

 either directly derived from them, or arise by the intermediation of a 

 median layer, which, when developed, takes an important part in 

 forming the tissues. 



4. When the whole of the animal series is considered, each of the 

 germinal layers is found to be, in certain creatures, capable of giving 

 rise to at least three, and perhaps to all the tissues ; the germinal 

 layers cannot, therefore, be regarded as histologically primitive 

 organs, 



5. In birds and mammals there is no primitive organ for the 

 formation of connective substance, blood, or vessels. 



6. The elements of tissues already formed have, as it seems, the 

 power of forming other tissues ; those of the heterologous neoplasms 

 are probably due to the remains of the embryonic cells or to elements 

 similar in character to them. 



7. There is no justification for the classification of the tissues as 

 archiblastic and parablastic, but, on the other hand, the old division 

 of the tissues under four primary types, as suggested by the author and 

 by Leydig, is still the most appropriate. 



Origin of the Mesoblast of Cartilaginous Fishes, f — C. K. 

 Hoffmann commences his essay with the description of a develop- 

 mental stage of Pristiurus metaholicus, which is a little later than 

 Balfour's stage B. In this there is as yet no trace of the notochord, 

 and the mesoblast is only beginning to be formed. There is still a 

 distinct medullary groove, and the intestine is in course of for- 

 mation. 



The study of a number of sections, here described from before 

 backwards, proves that the mesoderm forms a bilateral cellular layer, 

 which grows forwards and backwards. Anteriorly it commences 



* ZeitBclir. f. Wiss. Zool., si. (188i) pp. 179-21.3 (2 pis.). 



t Axch. Neeiiaud. gci. Ezact, et Nat., xviii. (1883) pp. 241-58 (2 pis.). 



