1877.] 175 [McCrady. 



ingly difficult, if not impossible, under the conditions of the earliest 

 changes in the egg. It would almost uniformly present the appear- 

 ance of mere spaces between the " cells " or " segment spheres," or 

 between the yolk and the egg membranes. 



It is supposed in this theory that such " Diaphane " as Carter pro- 

 posed to call this simplest form of protoplasm, plays an important 

 part in the early history of the embryo, constituting, perhaps, the 

 most active part of the protoplasmic mass of the protembryo, envel- 

 oping and taking up portions of the yolk, so as to form " cells " or 

 segment spheres, and also furnishing the common matrix in which 

 these when formed are embedded; filling also the cavity of von Baer 

 when present, and only gradually diminishing in quantity as the 

 tissues beeome more perfectly formed. 



13. The cases of total segmentation may be divided into those in 

 which a nuclear " embryonal vesicle " exists, and those in which 

 none has been observed. 



14. The latter case seems to be exemplified in the eggs of Geryo- 

 nia hastata as observed by Metschnikoff, where nuclei appear only 

 after the first cleavage ; and in those of Idyia roseola, where, according 

 to A. Agassiz, none appeared until the formation of sixteen spheres, 

 i. e., the fourth cleavage. The case of Eschscholtzia cordata investi- 

 gated by Kowalevsky, is particularly valuable on account of the care 

 taken to determine the presence or absence of a nucleus by a very 

 able observer, whose disposition was to believe in the persistence of 

 the germinative vesicle in all cases. He could find none before the 

 sixth cleavage, and then only in the small spheres which constitute 

 the " Bildungsdotter," our protembryo. Moreover Kowalevsky ob- 

 served in these eggs, as a preparatory phenomenon before segmenta- 

 tion, that the external protoplasm of the protembryo, " draws itself 

 together and rolls by means of these contractions the inner yolk 

 mass to one side or the other, in consequence of which contractions 

 it finally heaps itself up upon one of the two sides." These move- 

 ments are undoubtedly amoeboid, and they lead directly to the pro- 

 cess of segmentation of the enclosed yolk. The protembryo thus 

 massed on one side puts out two prominent wart-like processes, be- 

 tween which forms a depression. This depression increasing and 

 becoming a furrow, sinks gradually deeper and deeper towards the 

 centre, passing through which it finally reaches the surface at the 

 opposite side of the egg, which is thus divided into two segments. 

 On each of these the outer layer, or protembryo, now so arranges 



PROCEEDINGS B. S. N. H. — VOL. XIX. 12 NOVEMBER, 1877. 



