414 



JOHN BEARD, 



Figs. 9 and 10 are taken. In all three embryos there are small ex- 

 ternal gills upon all the five branchial arches, the neurenteric canal 

 is persistent, 45 — 46 muscle-buds are passing into the foundations of 

 the paired fins, and no traces of unpaired fins are yet present. 



As seen in text-figure E, the whole placode is as well-defined as 

 ever, but it has wandered, or been pushed, upwards and outwards by 

 growth of the hypoblast. It now lies on the level of the notochord, 

 and the plane of its surface is almost at right angles to what was 

 this plane during the gill-pouch-period. Any idea, that this placode 

 is, or has any connection with, the sensory placode of a branchial or 

 lateral sense organ, must be distinctly repudiated. As revealed in the 

 figures above referred to, leucocytes have now begun to form within 

 the placode, but, as will later on appear, their number is not great 

 during this period. 



In text-figure F the state 

 of afi'airs in embryos of the 

 important period of 25 mm is 

 given. At this time the neur- 

 enteric canal has closed, forked 

 muscle buds from some 46 

 somites for the paired fins can 

 be counted, the first traces of 

 ^^^S xO p. "5^ unpaired fins are present, but 



Ê Iv y^^^V ^^^^ contain no mesoderm, and, 



I ç^M ^^-^"^^^«»«-^ I \)\ finally, there are external gills 

 ^ ^ M^ J on all the branchial arches, 



^Igf^^^^"""™^^^ l^^^^r but those on the last two are 



^ ^^^^^ _ ^ From text-figure F it will 



be evident, that the placodes 

 are now almost on the level of 

 the notochord. They have in- 

 creased somewhat in thickness, but not much if at all in length. In 

 the plates there are several figures from this period of development 

 of the placodes; at this stage only Fig. 7 need be referred to in 

 illustration. Leucocytes are now fairly abundant in the placode, and, 

 indeed, their wanderings out from it, to be described at a later stage, 

 are now very obvious. 



It would have been an easy matter to have much increased the 

 number of text-figures of various periods, even to have carried them 



^■P■ 



F 



