Io8 MCMURRICH. [Vol. XI. 



some of the problems which have of late been exciting no little 

 attention from embryologists. Before passing on to consider 

 these facts, however, I wish again to emphasize what has 

 already been said as to the syncytial nature of the developing 

 Q.'gg up to the stage at which the cells completely separate 

 from the yolk, since it is to the existence of the syncytium 

 that the interest which attaches to the centrolecithal segmen- 

 tation as seen in the Isopods is due. In dealing with holo- 

 blastic ova in which there is apparently a formation of distinct 

 spherules, embryologists have been too apt to concentrate their 

 attention on the individual spherules, and to regard them as 

 more or less independent units and not as parts of a continu- 

 ous whole, and it is largely to the assumption of this stand- 

 point that the mosaic theory of development owes its existence. 

 There is, however, an increasing tendency towards the rejec- 

 tion of this theory and towards a return to the view long ago 

 (1867) enunciated by the botanist Hofmeister, according to 

 which the growth of the individual cells is determined by the 

 growth of the entire organism, an idea admirably expressed by 

 DeBary in the aphorism " Die Pflanze bildet Zellen, nicht die 

 Zelle bildet Pflanzen." Rauber ('S3) in an exceedingly sugges- 

 tive paper extended this idea to animals ; Heitzmann has 

 carried it to its full limits, and to-day it is upheld by such 

 authorities as O. Hertwig ('92), Whitman ('94), and to a certain 

 extent, Wilson ('93). 



In developing holoblastic and meroblastic ova it is difficult 

 to demonstrate actual continuity of protoplasm throughout the 

 various spherules, and so far as they are concerned the idea 

 just referred to rests upon a theoretical basis. With centrole- 

 cithal ova the case is different, and it is not difficult in the ova 

 of the species I have described to demonstrate the existence of 

 a syncytium. It is not improbable that such a condition will 

 be found in all centrolecithal crustacean ova, though up to the 

 present, so far as I am aware, it has been described by a single 

 author only, namely Samassa ('93) in his paper on the develop- 

 ment of the Cladoceran DapJmella. In this form, as well as in 

 the Isopods I have described, a considerable amount of yolk is 

 present, and there is no indication of total cleavage. There 



