864 SUMMARY OF CURRENT RESEARCHES RELATING TO 



scope ; " two complete series of vertical optical sections were obtained 

 by the camera lucida, one parallel with the longer, the other with the 

 shorter axis of the blastodisc." None of the ordinary hardening fluids 

 were found to be successful ; Kleinenberg's picro-sulphuric acid 

 causes the cleavage products to swell, and in many cases to become 

 completely disorganized. The best preparations of cleavage-stages 

 were obtained with osmic acid, followed by a modified form of 

 Merkel's fluid, 1 per cent, instead of • 25 per cent, of chromic acid 

 being used ; the eggs must be first killed by a weak solution of osmic 

 acid. When treated with alcohol, previous to cutting sections, the 

 egg-membrane should be broken or perforated. 



The teleostean ovum affords a beautiful illustration of what 

 Lankester has called " precocious segregation " ; the mature egg is 

 characterized by a very marked polar differentiation. The generaliza- 

 tion of Mark that the maturation spindle always lies in the axis of 

 the ovum may be carried further, and we may say that "it is highly 

 probable that the first cleavage-spindle invariably lies at right angles 

 to the axis of the ovum throughout the Metazoa ; and that therefore 

 the first cleavage-plane is always a meridian plane." The authors 

 discuss the apparent exceptions to this law that result from the 

 observations of other embryologists. 



With regard to the observations lately made by Pfliiger on the in- 

 fluence of gravitation, it is stated that, if an ovum be turned upside 

 down immediately after the appearance of the second cleavage, the 

 position of the third was not affected, as it was if the inversion was 

 effected an hour or more before the beginning of the first cleavage ; 

 the deduction made from this is that there is a corresponding internal 

 transposition of the active protoplasmic matrix of the ovum. 



Comparing the teleostean ovum with that of the frog, the authors 

 say " the central portion of the blastodisc represents the active por- 

 tion of the pigmented hemisphere of the frog's ovum; while the 

 marginal portion of the disk, together with the periblast [" parablast " 

 of His], represents the active portion of the unpigmented hemisphere." 

 The first cleavage-amphiaster was found to have a horizontal position, 

 at right angles to the ovum. 



The authors enter into some details with regard to the velocity of 

 cleavage, and state that the early cleavages are all introduced by 

 grooves running from the inner towards the outer surface of the 

 blastodisc. No discussion of the nature of the nuclei is entered into, 

 and the authors refrain from selecting any one of these possible 

 hypotheses as to the seat of the attractive power of that region of the 

 egg ; that is, namely, whether it resides in the nuclei, in a special 

 portion of the protoplasm intimately associated with the nuclei in 

 the process of division, or in both. Some of the accounts given by 

 Hoffmann are closely criticized. 



The origin of the endoderm has been variously described as being 

 from the periblast, or from delamination or invagination of the margin 

 of the blastodisc ; the authors state that they have positive proof of a 

 centripetal ingrowth of cells from the margin of the disk ; they hopo 

 to show that, contrary to the opinion of Balfour, the development of 



