Haddon — Note on the Blastodermic Vesicle of Mammals. 543 



states that he has not yet obtained a complete history of the first 

 cleavage spindle. The relation of the plane of first cleavage to the 

 axis of the ovum is therefore undetermined. If the first cleavage 

 divides the ectodermic from the entodermic pole, as supposed by 

 van Beneden, it is most probably equatorial. If further researches 

 prove that the cleavage begins with an equatorial groove, a very 

 important exception to the general rule will be established. If, on 

 the other hand, it turns out that the first cleavage is meridian, it 

 will be difficult to reconcile this fact with van Beneden's opinion 

 on the destination of the first two cleavage- spheres " (pp. 36-7). 



Remembering the great disturbance to normal segmentation, 

 which must result first from an increase in the amount of food 

 yolk, and secondly from its loss, it is not surprising that the 

 segmentation of the mammalian ovum apparently forms an ex- 

 ception to the general rule — an exception, moreover, which would 

 but emphasise the rule. 



Hoffmann (11) states that he has observed in the ova of certain 

 Teleosts (Scorpeena, Julis, &c.), the axis of the primary nuclear 

 spindle (archiamphiaster) is identical with that of the ovum; and 

 when two nuclei have been formed, both lie in this plane, the subse- 

 quent cleavage results in the division of the Qgg into two very un- 

 equal portions, an upper and smaller '.' arohiblast" (germinal disk), 

 and a very much larger " parablast" (yolk). " The arohiblast alone 

 segments, its nucleus is the parent of all the cleavage nuclei ; the 

 parablast doos not segment, nuclear division only takes place within 

 it ; it is altered into a multinucleated cell." Agassiz and Whitman 

 discuss Hoffmann's conclusions so keenly that until further evidence 

 is forthcoming we can only accept their view of Teleostean segmen- 

 tation. 



liyder (17, 18) supports Hoffman's conclusions, but without 

 giving a sufficiently precise account of his own observations on 

 the point. 



There can be but little doubt that the first immigration of blas- 

 topheres into the interior of the ovum (van Beneden's stage 3) 

 indicates the gastrula stage. It would further appear that this 

 immigration was asymmetrical, much as there is an asymmetrical 

 invagination of the hypoblast in telolecithal ova. 



Unfortunately, van Beneden's is the only account we possess 

 at the present time of the segmentation of the mammalian ovum. 



SCIEN. PROC, R.D.S. VOL. IV. PT. IX. 2 Z 



