16 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
“ plastidules,” nor so large and like one another as Spencer supposed 
his “ physiological units ” to he ; in size they are nearer to the smallest 
known organisms than to molecules, and they are as diverse as the 
characteristics of the organism are numerous. De Vries recognizes 
regular successions of cells “ from the fertilized egg-cell through the 
individual to the following generation,” -and distinguishes primary and 
secondary courses or tracks, of which the former run direct from germ- 
cell to germ-cell, while the latter are circuitous, giving the organism 
in many cases the power of asexual multiplication. The products of 
cell-division may be both on the germinal track (phylotic), or both in 
the body proper (somatic \ or one may be germinal while the other is 
somatic (somatarchic). The author does not allow the legitimacy of a 
hard and fast distinction between somatic and germinal cells. 
According to the theory of “ intracellular pangenesis,” the entire 
protoplasm is made up of “ pangenes.” Each characteristic of the 
organism has its special “ pangene.” Kepresentatives of all are found 
in the nuclei, while the body ef the cell contains for the most part only 
those which are essential to that cell’s activity. So many remain within 
the nucleus, and are active, for instance, in nuclear division ; so many 
must pass out into the protoplasm of the cell to unite with other 
“pangenes,” to multiply, and become active. The theory seeks to 
combine one of the fundamental ideas of Darwin’s pangenesis with the 
more modern conception of germinal continuity. 
Theory of the Mesoderm.*' — Prof. C. Eabl has been led by his inves- 
tigations on the segments of the Vertebrate head to consider the great 
jiroblem of the formation and differentiation of the mesoderm. His 
researches refer chiefly to embryos of Pristiurus, fowl and pigeon, and 
rabbit. 
I. The Formation of the Mesoderm. — (a) Eabl’s investigation of 
Selachian development leads to results essentially the same as Eiickert’s. 
That portion of the mesoderm which has its origin beside the chordal 
endoderm Eabl distinguishes as gastral, while that which arises from 
the endoderm of the invagination-margin is distinguished as peristomial, 
corresponding respectively to Eiickert’s axial and peripheral mesoblast. 
The two portions pass into one another at the posterior end of the em- 
bryonic rudiment. It is noteworthy that the peristomial mesoderm 
retains its connection with the endoderm longer than the gastral does. 
(b ) In the mesoderm of the chick-embryo at the end of the first day 
two portions are to be distinguished, that which arises from the head- 
process and that from the primitive streak. The two pass into one 
another at the anterior end of the streak. Except in the head-process 
and in the primitive streak there is never any connection between the 
mesoderm and the primary layers ; at the periphery the mesoderm stops 
with a sharp margin between ectoderm and endoderm. Eabl’s results 
are for the most part in accord with the well-known study of the germinal 
layers of the chick by Balfour and Deighton. (c) The author’s investi- 
gation of the blastoderm of the rabbit was less satisfactory, but his 
results seem to corroborate van Beneden’s conclusion that one portion 
of the mesoderm arises in the form of two symmetrical rudiments — 
* iMor[>hol. Jalirl)., xv (1889) pp. 118-252 (4 pis,). 
