114 



Dr. J. Beard. 



[May 16, 



spiracnlar cleft or rather its rudiment is a very early formation, 

 being developed long before hatching. The evagination to form it 

 reaches the formative epiblast and fuses with it, this fusion persisting 

 for a long time, but at no time can a lumen opening on the exterior 

 be detected. As "Wright has shown, a part of the spiracnlar cleft 

 persists in the adult in the form of a canal leading upwards and 

 forwards into the region of the periotic capsule. 



The first branchial cleft is formed long before the others and 

 before hatching. In newly hatched larvae it has a wide opening on 

 to the exterior. This opening soon becomes apparently less by the 

 growth of the operculum. 



The pneumatocele arises at a very early period and long before 

 hatching. It is a fold of the neural median hypoblast, and grows 

 backwards in length apart from connexion with the alimentary 

 canal. For some time the ventral part of the alimentary canal is 

 only filled in by yolk, by-and-bye the gut becomes shut in by in- 

 growths from the sides in the manner suspected by Balfour and 

 Parker. 



The somites, like the mesoblast from which they are formed, are at 

 first solid : they are long and narrow. As is usual, their mode of 

 formation is from before backwards ; but in front of the first one 

 formed, two others appear later on just behind the auditory capsule. 

 These latter are, as their fate shows, the two anterior somites of the 

 hypoglossus. Regarding head-somites, I prefer at present to say 

 nothing. 



The inner wall of the somites gives rise to muscle, and most of the 

 outer wall is converted into pigmented connective tissue. The 

 somites become much elongated, and their ends are constricted off as 

 buds to form the musculature of the paired and unpaired limbs. 

 Each end of each somite constricts off a single bud, which only 

 divides into two at a later stage than the twenty-first day. The 

 posterior paired fins only begin to develop towards the end of the 

 third week, and the muscle buds of the unpaired fins remain in an 

 embryonic condition beyond this period. 



Urinogenital Sy stein. — To form the pronephros there is a solid evagi- 

 nation of the mesoblast, uniting the somites with the somatopleure and 

 splanchnopleure. This arises early on the third day, and reaches 

 from the 4th to 8th or 9th somites inclusive. This probably fuses 

 with the epiblast, and at any rate a solid segmental duct is formed — 

 probably from tbe inner epiblast layer. This grows gradually back- 

 wards, having at first an indistinct form of termination, finally it 

 reaches the proctodasum, and becomes fused with it. 



After this stage, two additional mesoblastic somites are formed 

 behind the auditory vesicle. Then the first three somites behind the 

 ear are those of the hypoglossus, and, as van "Wijhe has determined 



