554 RECORD OF CURRENT RESEARCHES RELATING TO 



place. It appears that these spines are provided with numerous 

 marginal opposite hooklets, usually pointing towards the free end of 

 the spine, a structure which, so far as Dr. Young is aware, is unique 

 amongst the brachiopods, and to which he draws the attention of 

 palaeontologists, as perhaps similar structures may be found in other 

 brachiopods. 



Arthropoda. 



Structure of the Cerebrum and Retina in the Arthropoda.*— 

 Emil Berger finds that the optic ganglion of the Artlirofoda consists 

 of two parts, one of which is in direct relation to the facetted eye, 

 and goes to aid the layer of rods in making up the retina, while the 

 other part is more directly connected with the cerebrum, and is to be 

 regarded as a proper part of that organ. The retina consists of five 

 layers, which, passing inwards, are (1) The layer of rods, separated 

 by a fenestrated membrane from (2) the layer of nerve-bundles ; 



(3) The granular layer, where the nuclei are coarsely granular ; 



(4) The molecular layer, containing a finely granular substance, just 

 as in the Vertehraia ; and (5) The layer of ganglionic cells, which 

 were ordinarily found to be nucleated. Simplest in Artemia, the eye 

 becomes more complicated in the higher Arthropoda. The relations 

 of the retina to the cortical lajer of the cerebrum are particularly 

 interesting in the bee ; it is directly continuous with that of the optic 

 ganglion and the ganglionic portion of the retina, and this arrange- 

 ment seems to show that the last-mentioned structure is merely a 

 modified part of the cortical layer. It is interesting to compare this 

 arrangement with the well-known history of the development of the 

 Vertebrate eye. 



The pair of nerves which, in the bee, arise from the subcesophageal 

 ganglion are shown to take their origin from the supra-oesophageal 

 ganglion in great part, as it is but little of them that arises from the 

 cortical region of the lower ganglia ; this observation explains the 

 presence of the number of transverse commissures which were stated 

 to exist in this region by Leydig. In the Insecta the supra-oesophageal 

 ganglion is invested by a cortex containing ganglion-cells, while in the 

 higher Crustacea the ganglion-cells are arranged in a number of peri- 

 pheral and disconnected layers. 



Formation of the Blastoderm and of the Germ-layers in Insects.t— 

 Dr. Bobretzky, in his observations on this subject, draws attention to 

 the views of Weissmaun and of Metschnikoif. According to the former, 

 the formation of the blastoderm is preceded by the development of a 

 clear and almost homogeneous substance on the surface of the egg ; this 

 " blastema " encloses nearly the whole of the egg, and is readily distin- 

 guished from the dark granular yolk. In it there appear nuclei, which 

 are. later on, converted into the cells of the blastoderm ; and according 

 to Weismann, these nuclei are freshly developed structures, and not 

 descendants of the germinal vesicle. MetschnikofF, however, believes 

 that they owe their origin to the continual division of the germinal 



* ' Arb. Zool. Inst. Univ. Wien,' ii. (1878) p. 221. 

 t ' Zeitschr. wiss. Zool.,' xxxi. (1878) p. 195. 



