INTRODUCTION. 



the thinner portion of the mantle the name periblast was proposed. Blas- 

 todisc and periblast are thus names for two portions of one and the 

 same envelope, which invests the yolk-sphere. His formerly supposed 

 the periblast to be altogether foreign to the blastodisc in its origin, and. 

 in order to keep this distinction clear, introduced the terms archiblast and 

 parahlast. It has now been shown that the archiblast of His actually 

 includes both blastodisc and periblast, and hence the distinction he made 

 requires no longer recognition. If Hoffmann's observation on the division 

 of the first cleavage nucleus had turned out correct, there would still be 

 very strong ground for thinking that the ooplasm is composed of two very 

 distinct substances; but this observation, as we have shown and as Hoff- 

 mann himself now concedes, was entirely erroneous. This would settle the 

 question, had not Hoffmann, after giving up his "first equatorial" cleav- 

 age, appealed to a '"third," which he confidently asserts we have entirely 

 overlooked. Brook has taken similar ground. Now that our results are 

 presented with full illustrations, we may hope that the overlooked " third 

 equatorial " will soon find its grave with the " first." The following pages* 

 will be found to give a pretty complete history of the early history of 

 the periblast, and if it is not sufficiently complete to dispose of all these 

 anomalous " equatorial " cleavages, we are content to leave the sceptic 

 to the enjoyment of them. 



In no one of the many species of eggs described in the first part of 

 this memoir have we been able to find any indications that the periblast 

 formation differed essentially from the Ctenolabrus type. Our illustrations 

 have been drawn mainly from this species, not because the course of de- 

 velopment is more clearly marked here than in the others, but because 

 the supply of material could be more easily controlled. We have had the 

 eo-gs of at least a dozen different species in sufficient abundance for com- 

 parative study, and we have examined them with care, and can affirm with 

 the utmost positiveness that the story of one is the story of all. 



The observations of Professor Ryder on the periblast of the cod are 

 especially noteworthy, as they were made for the purpose of testing the 

 parablast theory of Hoffmann. On this point Ryder 3 remarks: "The 



* 



in 



The part devoted to cleavage, formation of periblast, and the development of the germ ring, is 

 course of preparation, and the text, with eleven additional plates will probably be issued toward the end of 



the current year. 



8 John A Ryder On the Development of Osseous Fishes, including Marine and Fresh-water Forms. 

 Cnited States Commission of Fish and Fisheries. Report of the Commissioner for 1885, p. 2, Washington, 



lss7 



