86 Transactions Texas Academy of Science. — 1906. 
are phylogenetically quite as old as those of the somatic layers though 
their arrangement into a particular tissue may be younger. From 
this germinal layer proceed first the definitive germ cells; second, 
cells later in segregation and definition than either the primary 
germ or somatic cells, having primarily the function of protecting 
and nourishing the germ cells, but some of which may later carry out 
more particularly somatic functions. The first distinction is, accord- 
ingly, between primary germ cells and primary somatic cells; the 
latter one between definitive germ cells and a second generation 
of somatic cells. 
The occurrence of a group of cells more or less intermediate in 
character between true germ cells and true body cells leads one to 
suppose that the distinction between the former kinds may not be 
so great as is generally supposed. This conclusion is strengthened 
by the knowledge that definitive germ cells exhibit the same general 
responses, and have the same main metabolic activities, as the somatic 
cells. It is known that germ cells by their chemical discharges exert 
a profound influence upon the body; and, on the other side, there 
is no reason to suppose that germ cells do not react to stimuli proceed- 
ing from the body. The simple fact that every case of embryonic 
development affords an example of germ cells becoming somatic is 
itself indicative that there is no unbridgable distinction between these 
kinds of cells. 
It would be out of place here to enter into a full discussion of the 
germinal cycles. It may simply be recalled that the line of the egg 
cells, the ovogenesis, essentially parallels the line of the sperm cells, 
the spermatogenesis, so that a particular generation in one can be 
compared with a particular generation in the other. On this point 
the intensive studies of recent years have confirmed the original con- 
clusions by Henking 16 and Oscar Hertwig . 17 The particular process 
in each germinal cycle that distinguishes it from any somatic one is 
that in a certain generation, that of the first spermatocyte and first 
ovocyte, there transpires an intimate pairing of correspondent chro- 
mosomes; and this conjugation is succeeded in one of the two fol- 
lowing cell divisions by a complete separation of every two chro- 
mosomes that had so paired. This is our present understanding of 
the question of ‘ 4 chromatin reduction.” The separation of the ele- 
ments of the bivalent pairs results from their position in the achro- 
matic spindle; their prior conjugation, however, must have a deeper 
significance than merely to bring about this separation. The long 
16 Untersuchungen ueber die ersten Entwicklungsvorgaenge in den Eiern der 
Insekten, 1890, Zeit. wiss. Zool., 49. 
"Vergleich der Ei- und Samenbildung bei Nematoden, 1890, Arch. mikr. Anat., 
36. 
