242 SUMMARY OF CURRENT RESEARCHES RELATING TO 



culaturo of Nephclis is feebly developed in comparison with that of 

 Hirudo and Aulostomnm ; both the longitudinal and transverse muscles 

 are very fine and thin. The provisional pharynx of the larva of 

 Ne2}hclis is not diflfcrentiatcd into pharynx ])roper and cesophagus, as 

 in Aulostomuvi. The author describes the formation of a provisional 

 or"an, the pharyngeal plate, but is not able to offer any suggestions 

 as to its function. 



The cephalic and trunk germs are paired, and unlike those of 

 Atdostomiun and Hiriulo, do not unite till a late stage in develojiment ; 

 just as in Nephclis, they form the rudiments of all the ectodermal and 

 mesodermal parts of the body, while the primitive epidermis, the 

 provisional musculature, and the body of the leech are all lost. 



Blastopore, Mesoderm, and Metameric Segmentation in 

 Phoronis.* — W. H. Caldwell was led to tlie essay now published by a 

 study of the origin of the mesoderm in Phoronis. In this remarkable 

 form the gastrula is formed by invagination ; the anterior end and the 

 future prseoral lobe are early indicated ; during the gastrulation the 

 extreme posterior end of the archenteric cavity becomes obliterated, 

 and represented by a fused solid mass of cells ; as the lips of the 

 blastopore touch they fuse with one anotlier, and this is eflfected from 

 behind forwards : the posterior portion may, as in some vertebrates, 

 be called the primitive streak, and the groove along the line of closure 

 the primitive groove. The ventral surface grows rapidly, and the 

 primitive streak thus becomes terminal. The anterior opening of the 

 blastopore remains open and becomes the mouth of Phoronis. 



Two pouchings which appear on either side of the blastopore 

 bud off a mass of endodermal cells on either side ; these arrange 

 themselves into a sac inclosing a cavity ; but they never communicate 

 with the cavity of the gut. On either side of the primitive streak a 

 few mesoblast cells are budded off from the cells that form it. The 

 hinder part of the groove gives rise, by deepening, to a pit which 

 grows out into two pouches, derived from cells that are homologous 

 with those that have already given rise to mesoblast : the fact that 

 in Phoronis the two ends of each mesodermic pouch are actually con- 

 nected by an intermediate cord of cells depends on the formation of a 

 primitive streak along the whole line of closure of the blastopore. 



The anus, like the mouth, is derived from the blastopore. 



" Given a gastraea already become bilaterally symmetrical by the 

 elongation of the blastopore, and the differentiation of anterior 

 inhalant and posterior exhalant currents, and in which the main 

 development of organs takes place around the mouth, so that the 

 mesoderm thus resulting comes to lie in development as two masses 

 of cells on either side of the body ; " the author proposes to show that 

 the elongation of a long axis is a possible cause of : — 



(1) The obliteration of tbe relation of blastopore to mouth and 

 anus. (2) The masking of the original mode of mesodermic forma- 

 tion. (3) Metameric segmentation. 



1. After noting the opinions of earlier naturalists, Mr. Caldwell 



* Quart. Joum. Micr. Sci., xxv. (1885) pp. 15-28 (1 pi.). 



