360 SUMMARY OF CURRENT RESEARCHES RELATING TO 



A similar kind of agreement is to be seen in the minute structure of 

 the nervous system ; in both the ganglia are made up of simple 

 small nerve-cells and nerve-fibres ; the latter alone form the perijiheral 

 tracts, and tliese are simple flat bauds. The resemblance in the mode 

 of forming plexuses is no less striking. The nervous plexus under 

 the ectodermal epithelium of the arms of the Brachiopoda is to be 

 compared to the stellate ganglionic cells connected with the neighbour- 

 ing nerve-trunks which are to be found under the epidermis of the 

 Cbtetognatha. As is well known, both the groups under discussion 

 exhibit the formation of an enterocoele ; both present a very symme- 

 trical structure, and have a spacious ccelom. The segmentation into 

 three metameres in the Brachiopoda is, in the adult, exhibited by the 

 gastroparietal and ileoparietal bands ; in both groups the second and 

 third metameres stand in relation to the reproductive organs. The 

 vasa deferentia of the Chaetognath belong as much to the type of seg- 

 mental organs as the infundibular ducts of the Brachiopod. In both 

 groups we find the enteric canal supported by dorsal and ventral 

 mesenteries ; in both, the central organ of the nervous system consists 

 of an oesophageal ring with an upper and a lower pair of ganglia. 

 From the latter arise two large nerve-trunks which pass backwards 

 and branch largely, without, however, giving any signs of a ladder- 

 shaped nerve-cord. The upper ganglion of the Brachiopod inner- 

 vates the arms and that of the Chtetognath the short tentacles. 

 Though there is a diflerence in the characters of the sensory organs, 

 there are indications of a degeneration of these parts in the Brachiopod. 

 The imj)ortant differences are the possession by the Brachiopod of a 

 shell and of a stalk ; but the former has been shown to be merely a 

 thickening of the cuticle, and the secondary value of the latter is 

 spoken to by the differences between the Testi- and Ecardines. 



Arthropoda. 



a. Insecta. 



Early Developmental Stages of Ovum in Insecta.* — Dealing first 

 with the Hymenoptera, A. Weissmann finds that in Bhodiies rosae 

 the shell of the mature egg is provided with a long peduncle ; the 

 germinal vesicle is replaced by a transparent nucleus, the first seg- 

 mentation-nucleus, devoid of a membrane. The yolk contracts, and 

 the nucleus becomes a whitish streak, which divides into two halves ; 

 these shorten, constituting the " polar nuclei " of the author ; they 

 perform amoeboid movements ; the jjosterior nucleus gives rise, by 

 elongation and fission, to about thirty nuclei. The nuclei now 

 migrate to the periphery and become surrounded by the yolk, which 

 forms a superficial layer to the ovum, and soon assumes a cellular 

 character, forming the blastoderm, the cells of which are uniform in 

 character, and all have a large nucleus directly invested by deutoplasm- 

 granules ; the periphery of the cell is formed of clear protoplasm. 

 The anterior polar nucleus buries itself in the yolk at the period at 



* Beitrage z. Anat. u. Physiul. als Festgabe Jacob Henle, Bonn, 1882. Cf. 

 Rev. Sci. Nat., ii. (1882) pp. 135-9. 



