History of a transient nervous apparatus in certain Ichthyopsida. 353 



nerve-process. That on the left is more difficult of analysis. It 

 would appear to be a simple fibril to which are applied three ganglion- 

 cells, two, and perhaps all, of which have their own fibrils running 

 to the centre. 



In an embryo of 13 mm (No. 198) the fourth and fifth branchial 

 pouches have not opened to the exterior, and 100 somites were counted. 

 The lens is in the form of a projecting knob of epiblast. The seg- 

 mental duct has not reached the cloaca. 



The transient nervous apparatus in this embryo is hardly as far 

 developed as in Nos. 410, 411 and 419. A second embryo of the 

 same size (No. 431) was preserved in Flemming's osmic mixture and 

 stained with haematoxylin. In it the lens is just being constricted 

 off from the epiblast. External gill-buds are appearing, the lateral 

 line thickening reaches only slightly posterior to the gill-region, while 

 the segmental duct extends as far as the cloaca. The paired fins are 

 obvious in the form of outgrowths of the side-wall of the body. The 

 transient nervous apparatus shows characters similar to those described 

 in No. 410, etc., and sub-epiblastic nerves are present, being in this 

 case deeply stained. 



3) Embryos of 17— 20 mm. 



R. batis No. 343 measured 17 mm and about 133 somites were 

 noted as present. The lens is still connected with the epiblast, but 

 the optic cup is invaginated, the anterior wall being now thicker than 

 the posterior. There are five gill-clefts open, in addition to the spiracle, 

 and on three arches external gill-buds. The paired-fin out-growths 

 are fairly large, their muscle-buds are constricted off from the myo- 

 tomes, and already partially within the outgrowths. The segmental 

 duct ceases in the skin on each side just near the opening of the 

 cloaca; it has as yet no connection with the latter. The oesophagus 

 is now solid for some distance posterior to the gill-region. 



The neurenteric canal still exists, but it is near the period of 

 closure — in the companion embryo (No. 344), which was accidentally 

 destroyed by one of my students, it was in process of closing. Trans- 

 ient nerves are first noticeable in No. 343 behind the pronephric region. 

 The first to appear are several fine nerves, spun by central ganglion- 

 cells, and roughly in correspondence on the two sides of the body. 



A little further back two nerves are met with, one on each side, 

 which in their general characters resemble those described in embryo 

 No. 410, but they have also stretches of ganglion-cells applied to 



