ZOOLOGY AND BOTANY. MICROSCOPY, ETO, 809 



of the head similarly appear as processes of the dorsal bundles of the 

 truuk. 



Hatschek reminds the reader that in his early essays on the develop- 

 ment of the nervous system of Annelids he has thus described it. 

 There first arises an anterior ectodermal thickening which extends back- 

 wards in the form of a cord on either side of the mouth, then forming 

 the oesophageal commissure ; and that then the process of thickening 

 goes on continuously in a backward direction, whence results the form- 

 ation of the two lateral cords of the ventral medulla. This statement, 

 which was based on observations made on Criodrilus, has been contro- 

 verted by Kleinenberg, who has studied Lumhricus. Hatschek, how- 

 ever, has found a confirmation of his views in the developmental 

 history of Polygordms and of Echiurus. He thinks that the commis- 

 sure is developed before the ventral medulla not only in Annelids, but 

 also in the trochophor-larvte of molluscs. Where there are no larvas, 

 but a direct development, there may be cases in which the cesophageal 

 commissure only secondarily unites the ventral cord and the frontal 

 plate. Even if Kleinenberg's observations are correct we have still to 

 face the question as to which mode of development is the more primitive. 

 Hatschek thinks that described by himself to be so, inasmuch as the 

 phylogenetic origin of separate nerve-centres, not connected with one 

 another by nervous connections, is highly improbable ; and Kleinen- 

 berg himself seems to have felt the difficulty. 



The author again disagrees with Kleinenberg as to the derivation 

 of the mesodermal structures of the head from the ectoderm ; he finds 

 that they are developed from the mesoderm of the trunk. In Poly- 

 gordius only the structures of the parietal lamella grow into the 

 head, and even then do not form a continuous layer, but appear as 

 separate processes of the muscular areas of the trunk. Balfour 

 attached great importance to Kleinenberg's observation that in the 

 head of Lumbricus a separate mesodermal cavity appears on either 

 side, and that these are connected with the corresjionding cavity of 

 the first primitive segment ; we may suppose that, phylogenetically, 

 two processes of the coelomatic sacs grow into the head, but against 

 this we have to put the anatomical relations of the cephalic cavity in 

 the Archiannelids. In these there is no dorsal or ventral mesentery 

 in the head, and the cephalic cavity is separated from that of the first 

 metaraere by a septum ; so that the conditions must be secondary and 

 not primitive. 



Development of Nematodes.* — M. P. Hallez has a note on the 

 devfclopmoiit of Ascaris megalocepJiala, the ova of which can bo very 

 easily cultivated ; development is etFected in from 15 to 25 days, and 

 takes jilacc more rapidly in damp air or oxygen than in water. In 

 distilled water, carbonic acid, hydrogen, or nitrogen it is slower. 

 Moderate elevation of the temperature aids development. 



The first segmentation-furrow is near the second polar globule ; 

 in stage 2 there is an ectodermal cell which is distinguished as 1 and 

 a meso-ondo-dcrmic cell e. 1 gives rise to 2 and e to e'. The form 



• ComptfB RcnduH, ci. (1885) pp. 170-2. 



