SUMMARY OF CURRENT RESEARCHES. 
405 
In another paper,* * * § the author discusses the yolk-syncytium of 
Selachians, with its undefined cell-areas and its uniform distribution 
of nuclei in a single layer. He shows how it differs at different points 
(marginal, central, &c.) at different times in development, and in different 
genera. 
Morphology of Vertebrate Head.f — Mr. H. V. Neal has studied 
this problem by investigating the nervous system in Squalus acanthias. 
He is unable to accept Locy’s “ neural segments ” as true segments, for 
they are irregular in size, inconstant in number, bilaterally asymmetrical, 
and without definite relation to structures known to be segmental. 
In the early stages of S. acantliias the neuromeres are local thicken- 
ings of the lateral zones, as well as dilatations of all the zones of the 
medulla. 
No structural conditions are presented by the myelomeres which are 
not reconcilable with the hypothesis that their existence is dependent 
upon the presence of the mesodermal somites. 
There was a primitive correspondence between neuromerism, meso- 
merism, and branchiomerism. 
The eye-muscle-nerves (3, 4, 6) are the serial homologues of ventral 
spinal nerves ; the ophthalmicus profundus is a segmental dorsal nerve 
belonging to metamere ii., while the oculomotor is its ventral root ; the 
trochlear is the ventral nerve of metamere iii., and the abducens 
represents the ventral nerves of metameres iv. to vii. 
There are five mesomeres alternating with six neuromeres in the otic 
and pre-otic regions ; probably eleven neuromeres are finally included in 
the head of Squalus. 
The author compares the mouth of Amphioxus with the left half of 
the mouth of Craniota, the first pair of permanent visceral clefts in 
Amphioxus with the hyomandibular clefts, and the eight visceral clefts in 
Amphioxus at its “ critical stage ” with the eight morphological clefts 
found in some Selachians and Cyclostomes. This report, however, gives 
only a hint of the scope of this important paper. 
Differentiation of Sex-Cells. + — I)r. v. Haecker distinguishes, in the 
development of the germ-cells of Cyclops , three differentiating processes : 
— (a) that associated with heterotypic division in the formation of the 
primitive germ-cells ; ( b ) that associated with the separation of paternal 
and maternal nuclear substance (Riickert) ; and (c) a third process (also 
seen in the differentiation of the genital rudiment) which seems like a 
disruption of nucleolar substance. 
Crosses between Different Breeds of House-Mouse.§ — Herr Georg 
von Guaita has sought to study the facts of inheritance by crossing 
different breeds of mouse, characterised by size, colour, markings, 
dancing power, and temperament. There is a general tendency to 
results which may be described as reversions to the original type. Thus, 
if the Japanese dancing mouse be crossed with the albino, the second 
generation consists of grey mice like the wild form. Continued in- 
* Tom. cit., pp. 91-110. 
f Bull. Mus. Harvard, xxxi. (189S) pp. 147-291 (9 pis.). 
f Ber. Nat. Ges. Freiburg, x. (1897) pp. 15-9. 
§ Op. cit., x. (1898) pp. 317-32. 
