ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 727 



and lateral amnion-fold are not formed by a rising up of tlie blastoderm 

 round the embryo, but are determined by the growth of the embryo and 

 its sinking towards the yolk-sac. In ontogeny the amnion owes its 

 origin to purely mechanical causes, the most important of which are the 

 weight of the embryo and the resistance of the zona in the presence of 

 the active forces of growth. The pro-amnion in the chick disappears at 

 about fifty hours' incubation, and subsequently contributes to the ventral 

 part of the head-end of the true amnion and to the wall of the yolk-sac. 



Fate of Amphibian Blastopore.*— Mr. T, H. Morgan, with Mr. E. C. 

 Applegarth, has examined the eggs of Amhiy stoma punctatum, Bana 

 halecina, and Bufo lentiginosus. Just before the disappearance of the 

 hypoblast beneath the epiblast, the blastopore elongates in the direction 

 of the primitive streak between the medullary folds. The anterior end 

 of the blastopore is directly continuous with the primitive streak ; except 

 in the region of the blastopore the medullary folds are widely separated, 

 but in it Aey almost unite, though at the posterior end a distinct opening 

 remains visible. In the next stage the neural folds have met everywhere 

 except at the posterior end, and at this point a cavity is left which seems 

 from surface-views to remain permanently open. A complete set of 

 longitudinal sections shows conclusively that, in Amhli/stoma, part of the 

 blastopore becomes the anus ; from a study of surface-views it was seen 

 that the elongated blastopore must I ave been in part arched over by the 

 closing walls of the neural tube. The question of any relation to the 

 neureuteric canal next suggested itself; some sections were seen in which 

 the neural tube dips in suddenly and becomes continuous with the cavity 

 of the mesenteron, and it became evident that the elongated blastopore 

 gives rise anteriorly to the neurenteric canal and posteriorly to the 

 permanent anus, while it closes in the middle part. Such a condition 

 is very transitory. In Bana halecina the blastopore closes and a new 

 anus is formed by a downward extension of the mesenteron meeting the 

 epiblast, and an opening appearing at the point of fusion. 



The hypothesis has already been advanced that the anus as formed 

 in frogs is an example of abbreviated development, and the author 

 believes that a study of Amhlystoma clearly indicates along what road 

 such a process has gone. If, instead of the blastopore itself elongating 

 and remaining open, the cells of its posterior wall should extend back- 

 wards, and subsequently should separate to form a new opening, a 

 condition such as is found in the frog and toad would be reached, and 

 would be merely an abbreviation of what takes place in AmhJijstoma. A 

 study of this last-named form suggests that the neurenteric canal is that 

 portion of the primitive blastopore which has been closed in or caught 

 by the folding over of medullary folds. If this rising up of epiblast is 

 a secondary phenomenon or an abbreviation of an earlier condition, "we 

 can see how by an elongation of the primitive blastopore its posterior 

 part escaped from being shut in, and remained as the anus of the adult. 



Penetration of Spermatozoa into Ova of Frog.t— M. J. Massart 

 has shown that the spermatozoa of the frog are retained by the surfaces 

 with which they come into contact, and that they seek to increase their 

 surface of contact. Dewitz has observed somewhat analogous phenomena 

 in the spermatozoa of the cockroach. Massart is able, however, to 



* Circ. John Hopkins Univ., viii. (18S9) pp. Hl-2. 

 t Bull. Acad. K. Sci. Bdg., xviii. (18S9) pp. 215-20. 



