122 MR. J. H. I5UDGETT ON THE 



Proio])terns are very interesting. The invafi;inaling- rim is often curved, as in fig. o n, 

 rather than straight, as in fig. 3, while later the invaginating rim may become som(>\vliat 

 V-sliaped, recalling a similar appearance in certain Ampliihia. The invaginjition 

 culminates in a crescentic blastopore. The yolk from the earliest stage onwards in 

 Protopterus is light green in colour. During segmentation the epiblastic pole of the 

 egg is pink, and this colour gradually replaces the green colour of the yolk, becoming, 

 however, paler as invagination proceeds. In the later stages, where the tissues are 

 becoming more transparent, the green-coloured yolk is again seen. 



As in Lepidosiren, the medullary groove arises far forwards and grows back to the 

 blastopore. In Protopteruti (PL X. fig. 6) the medullary folds, though wider in proportion 

 to the surface of the egg than in Lepidosiren {oj). cit. figs. 17 /> & 18 h), are not quite so 

 definite, but undoubtedly do encircle the blastopore in the same way just before they 

 close. From an external examination the blastopore seems to remain more widely 

 open after closure of the medullary folds than in Lepidosiren. 



PL X. figs. 7 t& 8, corresponding with Lepidosiren {op. cit. figs. 21, 22, & 23), show 

 a very similar origin of the brain, optic outgrowths, branchial and pronephric eminences, 

 but the pair of folds which will subsequently give rise to the mandibular and hyoidean 

 visceral arches is much more marked in Protoptems (PL X. fig. 7, M.H.). The 

 pronephric ducts have also an origin identical Avith Lepidosiren {op. cit. figs. 21 m. 

 22 m, 23 m). In PL X. fig. 8, which corresponds very nearly otherwise in development 

 with Lej^idosiren {ojj. cit. fig. 23 /), the whole embryo is not so flattened on the yolk, 

 the head and tail-fold being much more conspicuous. At this stage is seen the first 

 appearance of the crescent-shaped sucker (PL X. fig. 8, c.o.) first shown in Lejndosiren 

 {op. cit. fig. 24). 



In Lepidosiren the branchial arches arise on either side, first as one eminence (oj>. cit. 

 fig. 22), later three eminences (fig. 23); the last of these then splits into two, and 

 thus the four arches are formed. In Protopterus they arise first as one eminence 

 (PL X. fig. 7, hr.) ; later two eminences (PL X. fig. 8, hr. i. & ii., br. in. & iv.), these 

 then each split into two ^ (PL X. fig. 9, hr. i. ii. in. iv.), thus giving rise to the four 

 branchial arches. 



In Protopterus (PL X. fig. 9, M.S.), anterior to the four branchial arches, there may 

 be seen an indication of the mandibular and hyoidean arches, which in Lepidosiren 

 {op. cit. fig. 24) are represented by a single eminence. 



Protopjterus hatches about the stage of PL X. fig. 10, often a little later, in some 

 cases as late as PL X. fig. 11. Before hatching there appears to be a covering of cilia, 

 for particles in the fluid within the egg-capsule stream down the sides of the embryo 

 towards the tail end. At hatching the four pairs of external gills are a good deal in 

 advance of the gills of Lepidosiren, the developing pinnee being clearly seen. The rate 

 and direction of growth of the first pair of external gills is very diff'erent to that of 



' The cleavage of the hindermost eminence to form the Srd and -1th branchial arches occurs some-svhat 

 later than that of the foremost eminence. 



