1896.] Mr Mac Bride, Germinal Layers in Amphioxus. 151 



arises as successive pairs of hollow diverticula of the gut, each of 

 which gives rise to an adult myomere. When about half-a-dozen 

 pairs of somites have been formed the most anterior part of the 

 gut buds off two forwardly directed diverticula, which are at first 

 symmetrically placed and equal to one another, but of which later 

 the left becomes a small thick-walled pouch, which acquires an 

 opening to the exterior. The right on the other hand is converted 

 into a thin-walled vesicle, which extends forward and constitutes 

 the cavity of the snout. 



When rather more than a dozen somites have been formed the 

 remainder no longer arise as outgrowths of the gut, since a dorso- 

 lateral fold of the alimentary canal (by the division of which the 

 first somites were formed) becomes now completely separated from 

 the rest of the alimentary tract and constitutes a kind of primitive 

 streak, out of which the remaining somites are formed. 



Quite recently Basilius Lwoff (" Die Bildung der priinaren 

 Keimblatter und die Entstehung der Chorda und des Mesoderms 

 bei den Wirbelthieren," Bull, de Moscou, 1894) has given a 

 divergent account of the early development of Amphioxus. Ac- 

 cording to him the mesoblast is a dorso-lateral solid outgrowth of 

 the gut, in which the coelomic spaces (cavities of the myomeres) 

 appear later as secondary splits : it is true that the pressure of 

 the notochord on the mid-dorsal wall of the gut depresses it and 

 raises up the sides as two folds, but these have nothing to do with 

 the formation of the ccelom. 



The development of Amphioxus is exceedingly important from 

 a theoretical point of view, since in it we meet with the only 

 instance of a Vertebrate egg (outside the secondarily modified 

 eggs of the Mammalia), where the yolk is small in quantity and 

 evenly distributed, and where consequently the early developmental 

 processes are not impeded by its presence. Hence it is necessary 

 to interpret the development of other Vertebrates in terms of the 

 development of Amphioxus, and not vice versa: and in view of 

 this consideration, and also of the doubt in which Lwoff s paper 

 had enveloped our ideas on the subject of the formation of the 

 layers, I undertook a reinvestigation of the whole subject, and 

 hope to be able to publish my completed results this autumn. I 

 am greatly indebted to Mr Sedgwick for placing at my disposal 

 a large amount of valuable material collected in Sicily by himself 

 in 1889 and by Mr Willey in 1890. 



If we examine a section of a just completed gastrula we ob- 

 serve that it is roughly triangular in shape with the apex directed 

 ventrally, and that each of the lateral angles is produced into a 

 hollow ridge. In a slightly later stage we find that the cavity of 

 this ridge is shut off from the enteric space in front, whereas 

 behind it still opens into it. In a still later stage we find the 



