18 AUGUSTA ÄRNBÄCK-CHRISTIE-LINDE, NORTHERN AND ARCTIC 1NVERTEBRATES. 



to him (1. c. p. 466) the oviduct, which has the function of a sperm-duct, disappears 

 after the fertilization, and the ovum that remains enveloped in the outer follicle in 

 the mesoderm, wanders towards the peribranchial wall which encloses it in a pedun- 

 culate globular capsula in communication with the mesoderm. When the larva is 

 fully developed, the capsula bursts and the larva passes into the cloaca. 



Pizon's description agrees in the main with Della Valle's: »Apres la fécon- 

 dation le spermiduct se ferme; 1'ceuf est de nouveau, comme dans le jeune åge, 

 complétement enfermé entré la membrane péribranchiale et 1'ectoderme maternel 

 sans communication avec 1'extérieur — — — . Au moment ou 1'ceuf commence a se 

 segmenter, — — il a repoussé fortement devant lui la membrane péribranchiale 



externe et proémine dans la cavité péribranchiale, ou il est comme suspendu par un 

 pédicule de cette derniére membrane — — — » (1. c. p. 291. Pl. 9, fig. 94). Pizon 

 goes on to say that, when the larva is in an advanced stage of development, the 

 peribranchial wall of the parent animal and also the outer follicular epithelium — 

 which in his view still persists — bnrst on the mesial side whereupon the embryo passes 

 into the peribranchial cavity where it becomes attached immediately. Pizon who 

 by no means overlooked the fatal consequences of an open communication between »la 

 cavité sous-ectodermique parcourue par les globules sanguins et la cavité péribranchiale 

 parcourue par l'eau», gives the following description of the formation of the cup-shaped 

 structure: »Sur la coupe représentée par la fig. 76, on voit la membrane péribranchiale 

 maternelle accolée å ce qui reste du follicle externe et les deux membranes sont sou- 

 dées å leurs extrémités; on concoit que ces deux membranes se soient accolées 1'une 

 å 1'autre, fortement pressées qu'elles sont par le lar ve et méme qu'elles se soient 

 soudées å leurs extrémités, empéchant ainsi les globules sanguins de tomber dans 

 la cavité péribranchiale» (1. c. p. 296). Thus, in his view, the bottom of the cup- 

 shaped structure is formed by the persisting outer follicle. 



One brief glance at the figures illustrating this note, especially text-fig. 7, will reveal 

 the errors of the two writers last cited. For it appears to be established beyond doubt 



that the oviduct mediates the passage of the egg from the mesodermal layer 

 into the peribranchial cavity, 



that the fertilization and the development of the embryo take place in the 

 peribranchial cavity, 



that the outer follicle is thrown off before the egg passes into the peribranchial 

 cavity and degenerates long before the embryo has undergone its larval development. 



To those facts may be added that, in all probability, the bottom of the cup- 

 shaped structure is formed by the outer peribranchial epithelium in connection with 

 the contraction of the outer follicle thrown off. This supposition is moreover sup- 

 ported by the observation of Seeliger who states that the outer peribranchial wall 

 is folded to attach the embryo. There are other details in the formation of the 

 structure in question, and without due attention to them the process cannot be rightly 

 understood. Seeliger, however, . omitted to notice them and that is probably the 

 reason why no attention has been paid to Seeliger's statement by modern zoologists. 

 Mi*chaelsen seems to agree with Della Valle's and Pizon's view in respect to the 



