6 INTRODUCTION. 



might be extended to later stages, and hence our first memoir 1 bore this 

 serial title. Circumstances have prevented the continuation of the work, 

 and we have hitherto been unable to give more than a preliminary account 2 

 of the leading facts contained in this paper. If the unavoidable delay has 

 been any cause for disappointment, it cannot be said to have left us with 

 any abatement of interest in the subject. On the contrary, the field 

 covered by this memoir is one of growing importance, and the numerous 

 contributions that have followed our Preliminary Notice have increased 

 rather than diminished the value of the facts we have to record. The 

 majority of the latest writers have corroborated our statements on the 

 origin of the periblast, and our observations will now have less prejudice 

 to overcome than if they had been published at an earlier date. This is 

 one of the stiff-necked problems in vertebrate embryology, and one that 

 calls for a careful and connected study of the cleavage stages in the living 

 egg, in unbroken series of preparations mounted in Mo, and in sections of 

 preparations thoroughly oriented by a preliminary surface examination. 



Our material was abundant, and consisted mainly of the eggs of 

 Ctenolabrus, both naturally and artificially fertilized. Every stage has 

 been examined many times, in the several ways mentioned, and com- 

 pared with corresponding stages in other species. The eggs were pre- 

 served in various ways, the most successful of which have already been 

 detailed in our Preliminary Notice, pp. 28-30. 



Our account begins with the fresh-laid egg, and ends with the forma- 

 tion of the germ-ring. The phenomena displayed in the living egg have 

 received close attention ; and we would especially emphasize the impor- 

 tance of this side of the study as a means of arriving at a correct inter- 

 pretation of the conditions presented in sections and in mounted prepara- 

 tions. Among vertebrate eggs none are so favorable for this kind of 

 observation as pelagic fish-eggs, and it is to be hoped that this inviting 

 field of study, into which we have but just entered, will not long lie 

 fallow. 



In our Preliminary Notice (p. 42) we suggested one change in termi- 

 nology which seemed to be necessitated by the results of our study. The 

 discoidal thickening of the protoplasmic mantle of the egg, to which cleav- 

 age is at first restricted, we called with Haeckel the blaslorfisc, and for 



1 Alexander Agassiz and C. O. Whitman. The 1> ivelopment of Osseous Fishes. I. The Pelagic Stages 

 of Young Fishes. Mem. Mus. Comp. Zool., XIV., No. 1, Part I., September, 1885. 



2 On the Development of some Pelagic Fish Eggs. Preliminary Notice. Proc. Am. Acad. Arts and 

 Sciences, XX., August, 1S84. 



