356 SUMMARY OF CURRENT RESEARCHES RELATING TO 



middle portion, was functionally divided into two openings ; tte ner- 

 vous system was distributed in the ectoderm over the whole body, but 

 was probably especially concentrated on the oral surface. 



The view that an enterocoele was derived from one pair of archen- 

 teric pouches has been given up since Hatschek discovered that, in 

 AmpMoxus, a series of diverticula are formed ; the similarity between 

 the embryo of AmpMoxus and an Actinozoon polyp suggests that the 

 mesoblastic somites of segmented animals are derived from a diplo- 

 blastic coelenterate-like ancestor with folded gut-walls, and the 

 Coelenterata differ only from segmented animals in that the alimentary 

 or archenteric pouches (mesoblastic somites) and the alimentary canal 

 do not become separate. Connected with this absence of a distinct 

 ccelom is the low state of differentiation of such coelomic structures 

 as the excretory organs, and the absence of a separate vascular system. 



Sedgwick is of opinion that the excretory organs were not at first 

 used for the excretion of nitrogenous waste products, but for the 

 riddance of the undigested and solid excretory products, and that this 

 process was at first intracellular. Pores are certainly found in the 

 Diploblastica ; the circular canal of the Medusae may be easily con- 

 ceived as becoming the vertebrate segmental ducts, and a temporary 

 longitudinal canal has been observed by Hatschek in Polygordius. The 

 gill-slits are regarded as being serially homologous with nephridia. 



The author concludes the first half of his paper by pointing out 

 that his object has been to show that the majority of the Triploblas- 

 tica are built upon a common plan ; that that plan is revealed by a 

 careful examination of the anatomy of Coelenterata ; that all the most 

 important systems of organs of those Triploblastica are found in a 

 rudimentary condition in the Coelenterata, and that all the Triploblas- 

 tica referred to (Annelida, Arthropoda, Mollusca, Vertebrata, Balano- 

 glossus, Sagitta, and the Brachiopoda), must be traced back to a 

 diploblastic ancestor common to them and the Coelenterata. 



In the second part Sedgwick applies his hypothesis to the just 

 mentioned groups ; from the ideal ancestor, a little more advanced 

 than the common Coelenterate ancestor, two stocks branched off. In 

 one, the Invertebrata, the mouth and anus (which soon became sepa- 

 rated) remained on the neural surface, and a praeoral abneural lobe 

 was developed which, being carried first in movement became specially 

 sensitive. In the other stock the mouth and anus became terminal 

 and a projection on the neural surface of the body overhung the mouth ; 

 this is the stock of the Vertebrata and of Balanoglossus. The further 

 changes in the two stocks are next discussed, but space will not 

 permit of our following the author. 



In conclusion there are some observations on the structures known 

 as primitive streaks ; these are always connected with the formation 

 of the mesoblast, are not found in free larvse, are always median 

 and unpaired, are always caused by rapid proliferation of cells, and 

 vary in position in different animals. The differences which obtain 

 are at present very difficult of explanation, but one is perhaps to be 

 found in the supposition that owing to the early specialization of the 

 enteron the mesoblast has to be formed in some other than the primi- 



