ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 799 



in size, and sending ont pseudopodioid processes into the yolk ; their 

 nuclei lose their nucleoli and become ellipsoidal in form ; their cells 

 sink into the yolk and become covered by the neighbouring ecto- 

 dermal cells ; as they sink they increase in size ; this is a very different 

 history from that of some other insects, e. g. some Lepidoptera, and 

 is the direct opposite of what has been observed by Bobretzky in 

 Oniscus mnrarius. In other words, we may, among the Arthropoda, 

 have a centrifugal mode of formation of the endoderm, or a centripetal 

 one. In the scorpion we find an intermediate stage, for in it, after 

 the cleavage of the blastoderm into ecto- and mesoderm, large granular 

 cells appear beneath the latter, and form the rudiments of the 

 endoderm. 



The author directs attention to the fact that the mode of formation 

 of the mesoderm in worms is quite different from that which obtains 

 in insects, but is like what is found in molluscs ; for in both the 

 mesoderm only arises from large mesoblasts, which are derived from 

 the ectoderm ; a similar process has been found in Gryllotalpa, and 

 appears to be the first instance of elements homologous with the 

 mesoblasts of worms being found in insects. The development of 

 the myoblasts is next described. 



In the middle line there appears a neural groove which divides 

 the nervous thickening into two lateral halves ; this extends unin- 

 terruptedly from one to the other end of the embryo. The myoblast 

 which lies under the whole of the germinal disk, becomes divided 

 into two layers, and it is not till these appear that we observe the 

 segmentation of the myoblast. The formation of the " dorsal organ " 

 is described, and it is concluded that it is nothing else than a 

 stopper to the " nabel " which would, otherwise, be oj)en, as it is iu 

 the Lepidoptera, where, however, the orifice is extremely small. 



The inner layer of the myoblast is in the thoracic region, exca- 

 vated into a spacious cavity ; it gives rise to the muscular system of 

 the limbs, the two muscular bands which lie on either side of the 

 ventral nerve-cord, and to the so-called abdominal diaphragm which 

 separates the sinus which incloses the nervous system from the 

 ccelom. The fate of the outer half of the myoblast is connected with 

 the development of the heart, the histiogenesis of which is carefully 

 described. 



After the heart the author passes on to the nervous system ; there 

 are only seventeen ganglionic masses, the eighteenth segment having 

 none. The pair of sympathetic ganglia cannot be brought into relation 

 with any special segment. After the disappearance of the germinal 

 groove a thickening arises in its place, whicih consists of cylindrical 

 elements and rounded cells ; the nervous system becomes differentiated 

 from the ectoderm from before backwards. No commissures appear 

 until the fibrillar part of the nerve-cord has become developed. The 

 author agrees witli Tichomiroff in thinking that each nerve-cell is a 

 simple ectodermal element. Grt/Uotalpa, from the size of its cells, is 

 an admirable object for investigations of this kind. The cephalic 

 nervous system consists of two primitively separate lobes ; by an 

 incision the optic part of the brain becomes constricted off; the two 



