42 PROCEEDINGS OF THE AMERICAN ACADEMY 



From the time when the pronuclei appear to the moment when the 

 first cleavage-groove begins, is a short period of not more than thirty 

 minutes ; but it is a period of special interest in the history of the 

 nuclei, as will presently be seen. We have watched this period 

 through twice on artificially fertilized ova, and not less than a dozen 

 times on ova taken directly from the sea. The Newport Labora- 

 tory is so near the place from which most of our material was derived, 

 that we found no difficulty in obtaining ova from five to tea minutes 

 after fecundation. 



The polar disc, to which cleavage is at first restricted, has received 

 a variety of names. It is the " Anschwellung" of Rusconi; the 

 " Bildungsdotter " of Reichert; the "ampoule du germe," or "vitellus 

 formateur " of LerebouUet ; the " archiblast " of His ; the " blasto- 

 disc " of Ilaeckel ; the " disque germinatif," " germinal disc," " Keim," 

 of various writers ; and is the exact equivalent of the " discus pro- 

 ligerus " (Von Baer) of the chick. It gives rise to the " cap," "calotte 

 blastodermique," or " blastoderm," of later stages. "We shall employ 

 for this portion of the ovum the term blastodisc, since it is a word 

 now in general use, and is not burdened with any of the theories 

 connected with the use of the name " archiblast." 



The thinner portion of the protoplasmic mantle of the ovum is 

 also known under various names. It is called "membrane vitellaire" 

 by Vogt ; " Dotterhaut," by Oellacher ; " membrane interne," " feuil- 

 let muqueux," by LerebouUet; "couche intermediaire," by Von Bara- 

 beke ; " intermediate layer," by Von Beneden ; " parablast," b}' His ; 

 "pellicle," " yolk-hypoblast," by Ryder. As the name "parablast" 

 has been used in very different senses by His, Klein, Hoffmann, Wal- 

 deyer, and others, it seems best to employ a new term, which is at 

 least free from confusing associations. For this portion of the ovum 

 we propose the ira.me periUast, — a name which has the advantage of 

 not prejudging the question of the origin and destination of the part 

 it designates. 



Blastodisc and jjeriblast are simply names for two portions of one 

 and the same envelope, which invests the vitelline sphere. In all the 

 ova we have studied the blastodisc occupies the lower pole ; but for 

 purposes of comparison it will be convenient to speak of this as the 

 upper, or ectodermic pole; and of the opposite, as the lower, or ento- 

 dt-rmic pole. The vertical line joining these two poles is the axis of 

 the ovum. 



Tlie peculiarities in the development of the ovum will be best under- 

 stood by regarding it as an amphibian ovum, in which the active pro- 



