458 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
that “ the symmetrical arrangement of cells in the blastoderm is the mere 
outward expression of a physiological bi laterality which already exerts 
control over the life of the organism.” 
The periblast nuclei arise in Serranus as Agassiz and Whitman have 
described in Ctenoldbrus, viz. from the marginal cells of the blastodisc. 
During the formation of the periblast the superficial layer of the blasto- 
disc becomes differentiated as the DeckscJiicht or “ epidermic stratum,” 
which is gradually flattened into a thin membrane. The blastodisc 
undergoes a change of shape preparatory to the invagination. The 
appearance of the sub-germinal cavity (a late phase of the segmentation 
cavity) between the blastoderm and the periblast, the formation of the 
J Randwulst, the process of ingrowing, the significant centrifugal delami- 
nation in the Bandwulst, are then described. 
After the germ-ring is completed the growth of the blastoderm round 
the yolk is much more rapid than in earlier stages. The embryonic 
shield in the region of the posterior pole becomes sharply marked off. 
The ectoderm of the shield forms a thickened plate passing at the edge 
into the thin ectoderm of the non-embryonic area. Over the greater part 
of the shield the primary hypoblast consists of two distinct strata of 
flattened cells. An interlocking in the middle line is the first step in 
the formation of the notochord. The thickening of ectoderm to form the 
neural cord, beginning in the future head region ; the development of 
the notochord and secondary layers ; and the formation of the primitive 
streak and closure of the blastopore are fully described. 
After a careful study, Mr. Wilson feels safe in saying that the lateral 
sheets of endoderm grow under the chorda cells and meet in the middle 
line, thus completing the layer. At the time of the closure of the 
blastopore (29 hours) the mesoblast consists of two thick lateral masses 
thinning away at the sides, but not exhibiting the two-layered arrange- 
ment which in the trout prefigures the differentiation into somatopleure 
and splanchnopleure. In opposition to Hertwig, Wilson agrees with 
other students of Teleostean development in maintaining that the 
mesoderm arises by delamination. 
The alimentary canal is formed from the simple endoderm lamella 
by a process of folding along the median line. The fold is converted 
into a tube by the meeting of its lower edges. There is a solid postanal 
gut formed as a thickening of the endoderm lamella, not as a fold. At 
the end of the postanal gut is Kupffer’s vesicle which is formed in 
essentially the same way as the permanent gut. Kupffer’s vesicle and 
the entire postanal gut atrophy. In the interpretation of Kupffer’s 
vesicle, Wilson substantially agrees with J. T. Cunningham — it is the 
terminal part of the archenteron. 
Passing over the description of the development of neural chord, 
surface ectoderm, eye, &c., we may emphasize the interest of Wilson’s 
observation that one ectodermic Anlage gives rise to the ear, a functional 
branchial sense-organ, and the organs of the lateral line. This is not 
to be regarded as of phylogenetic significance, but as a convenient method 
of forming these organs, which the embryos of certain animals have 
adopted. The fact serves, however, to emphasize in a striking way the 
homology between the organs. In many respects Wilson here corro- 
borates Beard’s conclusions. 
