ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 603 



mesoblast develops, in characteristic crustacean fashion, independently 

 of the somites, into which the parietal sheath becomes subsequently 

 segmented. The growth of the mesoblast, the appearance of the 

 lateral appendage-rudiments and alternating ganglia, the mesoblast 

 growth round the dorsal vessel, and the progressive differentiation 

 of the organs is then described in detail for larvaa at various successive 

 stages. The presence of a hitherto overlooked sense-organ between 

 the brain and the frontal eye is noted. 



II. The segmentation of the mesoderm and the differentiation of 

 ectodermic and mesodermic organs. — The posterior portion of the 

 mesoderm band, Prof. Clans calls the budding zone. In front of 

 this, cross bands of two cells abreast are formed, raj)idly growing, 

 by the division of these cells, into thick mesoderm somites, in which 

 three regions become more or less clearly distinguishable — the dorsal, 

 forming the heart-rudiment and dorsal muscles — the median or lateral 

 forming the musculature of the joints — the ventral forming muscle 

 and neurilemma. The connective tissue of the horizontal septum, 



. the blood-corpuscles, &c, are similarly derived. The first rudiments 

 of the appendages are due to ectoderm proliferations which soon 

 become associated with mesoderm. It is worth noting that except in 

 the testicular cells no nuclear spindles were seen, so that direct 

 division is regarded as normal. The rudiments of the ganglia are 

 at first separate ectodermal thickenings, which, sinking in, become 

 secondarily connected. In no case does a nerve arise as a secondary 

 outgrowth from the nerve centre ; only the frontal sense-organ seems 

 to develope in this way from processes of the frontal nerve-cells of the 

 cerebral ganglion. The grouping of the muscle-cells in definite 

 direction is then described. 



III. The formation of regions and the number of the segments. — The 

 impossibility of establishing exact homologies between the variable 

 adaptive modifications into head, thorax, and abdomen in different 

 types is emphasized. Entering into a detailed discussion of the 

 number of segments, Prof. Claus criticizes the famous experiment of 

 Schmankewitsch, maintaining that there is really no difference in the 

 number of abdominal segments, while there are indeed numerous 

 distinctions between the two genera. 



IV. Integument, connective-tissue, and fat-bodies. — Branchipus 

 affords beautiful illustration of the chitinous modification of part of 

 the protoplasm of the hypodermis cells to form not only the cuticle, 

 but the sinews and internal plates. The three layers described in 

 Decapods are not differentiated in Branchipus, where the external 

 structureless cuticle is generally alone discoverable, though in some 

 regions a deeper fibrous layer can be detected. It seems sometimes 

 as if the connective-tissue structures which Tullberg described, in 

 the lobster, between the chitinogenous cells and the subjacent 

 connective tissue, were really present, but this appearance is due to 

 a non-nucleated internal cuticle, resulting from the modified basal 

 protoplasm of the epithelial cells. Connective fibres abundantly 

 distributed in the joints are also products of the chitinogenous cells 



