592 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
extended, the cells themselves moving in the same direction ; the pigment 
of the cells which surround the pedicel advances towards the cornea, and 
reaches as far as the outer pigment zone, so as to form a continuous layer 
of pigment which extends from the distal extremity of the retinophore as 
far as the basal membrane. 
As the results now reached support those of Patten there does not 
seem to be any justification for regarding the eyes of Branchipus, 
Gammarus or Astacus as compound eyes ; they are, rather, simple eyes 
the cornea of which is differentiated in a special manner, and the pigment- 
cells of which are grouped more regularly than in Vertebrates, where the 
adaptation of the optic organ to the changes in the surrounding media 
are produced by means of special organs, which are wanting to the 
Crustacea. 
In the Crustacea this adaptation is effected by the movement of the 
granular pigment, and the pigment-cells. The eye of Gammarus , by 
the possession of a smooth cornea and an undifferentiated hypodermis — 
characters which distinguish the simple eyes of Arthropods — as well as 
by the structure of its calyx and style, approaches the eye of lower Crus- 
tacea ; while, by the possession of pigment-cells which are not found in 
them, it affords an intermediate stage between the ocelli of Arachnids and 
the larval form of Arthropods on the one hand, and the so-called 
compound eyes of Crustacea on the other. 
Notwithstanding some differences in detail the present researches, 
taken with those of Madlle. Stefanowska on Insects and those of Engel- 
mann on Vertebrates, allow us to formulate the following generalization ; 
in eyes exposed in darkuess the pigment tends to occupy the smallest 
surface, while in light it spreads considerably, so as to protect the 
receptive elements against the influence of light. 
Development of Mesoderm of Crustacea.* — M. L. Houle has a note 
on the development of the mesoderm of Crustacea, and that of the organs 
derived therefrom. His investigations have been carried out on Porcellio 
scaber and Palsemon serratus. At the time when the cells of the blasto- 
derm are increasing on the medioventral line to produce the nerve- 
centres, and at the sides of the anterior end of the body to give rise to 
the foundations of the endoderm, two new bands of proliferation appear 
on either side of the ventral nervous band. The peripheral blastoderm 
becomes ectoderm ; the central cellular mass represents the mesoderm ; 
the cells of this mass are converted into muscular fibres. Similar cell- 
multiplications are found in most of the remaining portion of the 
blastoderm, but they are less active ; they only produce elements which 
penetrate into the subjacent yolk, and gradually destroy the nutrient 
materials which it contains. These elements correspond to the vitelline 
cells of authors, as to which opinions of such different kinds have been 
expressed. They are all derived from the blastoderm alone, and are to 
be regarded as part of the mesoderm of the body. 
The further development of the mesoderm is on the mesenchymatous 
type ; the mass in each young appendage commences to form a central 
cavity, two or three being sometimes juxtaposed ; the cells around the 
cavity break away from their neighbours, and become free in the interior. 
This process of dissociation leads to the formation of a network of meso- 
* Cornptes Ecndus, cxiii. (1891) pp. 153- 5. 
