762 SUMMARY OF CURRENT RESEARCHES RELATING TO 



however, are not thus transferred. The cellular elements detected by 

 Goette on the floor of the germinal cavity, and thence seen to wander, 

 at an early period, into the germinal wall, must not be referred to the 

 white yolk itself. Certainly the latter contains cells, but these, as 

 above shown, have come from the cleaved germ, to which they return. 

 The white yolk has no direct share in the formation of the embryo. 



It is well to be prepared with a definite answer to the question — • 

 what is a germinal layer ? The cells constituting a primitive lamina 

 must fulfil two conditions ; — they must arise directly from the germ, 

 and possess their own peculiar properties. But two such laminge can 

 be demonstrated, the ectoderm and the endoderm. We should not 

 reckon as primitive layers either (a) the aggregate of residual cells 

 (inner pro-embryonic layer of Goette = first inner lamina of Remak) 

 which lies beneath the ectoderm of the egg before incubation or (b) 

 the mesoblast, which consists, when first formed, merely of undiffer- 

 entiated cells of the germ, after its ectoderm and endoderm have 

 separated. The admission of the middle-germ to the rank of a funda- 

 mental embryonic lamina renders impossible all attempts to settle the 

 homologies of the primitive layers among the different classes of 

 animals. But if we accept two primary layers only, this difficulty is 

 removed at a stroke. 



Thus, in like manner, may we classify the tissues from an embryonic 

 point of view. They are either simple or compound. The simple 

 tissues are ectodermic, endodermic or mesoblastic. The compound 

 tissues are formed by the union of cells from the middle germ, or 

 their derivates, with cells from the ectoderm or endoderm. This 

 union is closer and takes place at an earlier stage of differentiation in 

 the case of the striped muscle. of vertebrate animals than with their 

 cerebro-spinal system. We know that in some coelenterates at least 

 the muscular and nervous tissues are purely products of the ectoderm. 

 There is still doubt as to the origin of many of the so-called endothelia. 



Dimensions of Histological Elements.* — W. Krause gives a list 

 (in 26 pp.) of the dimensions of the various histological elements, 

 classified under different headings, such as Connective Tissue, 

 Muscular System, Nervous System, &c. 



" Nervous System " (e. g.) is subdivided into Nerve Tissue, 

 Spinal Cord, Brain, Peripheral Nerve System, and Nerve Endings, 

 and the 1st subdivision is dealt with thus : — 



Nerve fibres, 0' 0018-0 -013 (Krause), 0-001-0-02 (Kolliker) 

 thick. 



Olfactory fibres, 0-0038-0-0068 wide, 0-0018 thick. Nuclei, 

 0-0068-0-0113 long (Frey). 



Pale nerve-fibres, 0-0017-0- 0027 thick (Krause); (in Mammals) 

 0-0033-0-0056 wide, 0*0013 thick; Nuclei, 0-006-0-015 long, 

 • 0045-0 • 0067 wide (Kolliker). 



Medullated nerve-fibres and ganglion-cells are dealt out in a similar 

 way. 



* Krause, W., 'Nachtr'age zum er3ten Bande des Handbuches der Meuscli- 

 lichen Anatomie von C. F. T. Krause (3rd Aufl.).' 8vo, Hannover, 1881, pp. viii. 

 and 170 (1 pi. and fel figs.). 



