92 PEOP. ALLMA.K ON THE 



measuring about three fourths of an inch in length, all the meri- 

 dional vessels had opened into the circular canal, while their 

 cseeal offsets had become numerous and ramified. The series of 

 swimming-plates, however, had not yet attained their entire 

 length, while the generative organs had not made their appear- 

 ance, and the processes which spring from the margin of the 

 tentaculiferous disks were still nearly simple. 



For reasons already mentioned this paper was unaccompanied 

 by figures ; and I have therefore probably no reason to complain 

 that the points in which it anticipates the results of subsequent 

 research have been either overlooked, as by Eol, who does not 

 seem to have known of its existence, or to have been misunder- 

 stood, as by Kowalewsky, who, though aware of the existence of 

 the paper, does not seem to have thought himself called upon to 

 cite it. I am indebted to Alexander Agassiz first, and to Chun 

 afterwards, for having amply recognized it. 



Kowalewsky has studied the development of several species of 

 Ctenophora, among which he has especially attended to that of 

 JEscJiscJioltzia cordata. His account of the structui'e of the egg in 

 this Ctenophore agrees with my own in recognizing not only an 

 external structureless membrane, but a diiferentiation of the vitel- 

 lus into a peripheral and a central portion. The latter, however, 

 he describes as composed of fat-globules ; and he regards it as 

 performing the function of a food-yolk (Nahrungsdotter), while 

 the peripheral portion constitutes the formative yolk (Bildungs- 

 dotter),and aff'ordsthe proper foundation for the embryo. With 

 this view I am unable to agree. From my own observations, cited 

 above, it follows that the whole mass of the ovum participates in 

 the cleavage, and contributes directly to the formation of the 

 embryo. Kowalewsky has seen the peripheral layer exhibit 

 protoplasmic movements of contraction previously to the com- 

 mencement of cleavage. 



The remarkable and eminently characteristic feature in the 

 egg-cleavage of the Ctenophora, which, as described above, results 

 in an outer layer of minute cleavage- spheres, by which the cen- 

 tral mass of larger, more slowly segmenting spheres becomes 

 gradually enveloped, has also been well observed by Kowalewsky, 

 who gives fuller details of the process than it was possible to 

 embrace in the short abstract which I had myself published. 



He describes the protoplasm of each of the large central spheres, 



