280 CELL-DIVISION AND DEVELOPMENT 



tunicate Ciona the usual relation is reversed, the polar bodies being 

 formed at the vegetative (i.e. deutoplasmic) pole, which afterwards 

 becomes the ventral side of the larva. In Ascaris Boveri's observa- 

 tions seem to show that the position of the polar bodies has no con- 

 stant relation to the adult axes, and Hacker describes a similar vari- 

 ability in the copepods (Fig. 130). My own observations on the 

 echinoderm-Qgg indicate that here also the primitive egg-axis has 

 an entirely inconstant and casual relation to the gastrula-axis. It 

 may perhaps still be possible to show that these exceptions are only 

 apparent, and the principle involved is too important to be accepted 

 without further proof. As the facts now stand, however, they seem 

 to admit of no other conclusion than that the relation of the primitive 

 egg-axis to the adult axes is not absolutely constant, and may in par- 

 ticular cases be variable. To admit this is to grant that this relation 

 is not of a fundamental character, and that the axes of the adult are 

 not predetermined from the beginning, but are established in the egg 

 in the course of development. 



(b) Axial Relations of the Primary Cleavage-planes. Since the 

 egg-axis is definitely related to the embryonic axes, and since the 

 first two cleavage-planes pass through it, we may naturally look for a 

 definite relation between these planes and the embryonic axes ; and 

 if such a relation exists, then the first two or four blastomeres must 

 likewise have a definite prospective value in the development. Such 

 relations have, in fact, been accurately determined in a large number 

 of cases. The first to call attention to such a relation seems to have 

 been Newport ('54), who discovered the remarkable fact that the first 

 cleavage-plane in the frog s egg coincides with the median plane of the 

 adult body ; that, in other words, one of the first two blastomeres 

 gives rise to the left side of the body, the other to the right. This 

 discovery, though long overlooked and, indeed, forgotten, was con- 

 firmed more than thirty years later by Pfliiger and Roux ('87). It 

 was placed beyond all question by a remarkable experiment by Roux 

 ('88), who succeeded in killing one of the blastomeres by puncture 

 with a heated needle, whereupon the uninjured cell gave rise to a 

 half-body as if the embryo had been bisected down the middle line 

 (Fig. 131)- 



A similar result has been reached in a number of other animals by 

 following out the cell-lineage ; e.g. by Van Beneden and Julin ('84) 

 in the egg of the tunicate Clavelina (Fig. 126), and by Watase ('91) 

 in the eggs of cephalopods (Fig. 127). In both these cases all the 

 early stages of cleavage show a beautiful bilateral symmetry, and not 

 only can the right and left halves of the segmenting egg be distin- 

 guished with the greatest clearness, but also the anterior and poste- 

 rior regions, and the dorsal and ventral aspects. These discoveries 



