EMBRYOLOGY OF THALASSEMA MELLITA 171 



ond cleavage. At this time one of the polar bodies begins to 

 sink in between the blastomeres and later passes into the 

 cleavage space. At the second cleavage the spindles are not 

 parallel, but are so placed that, when the cleavage is completed 

 the B and D quadrants meet at the lower pole, and the A and 

 C quadrants at the upper, forming two polar furrows which are 

 at right angles to each other (Text-Fig. i, B^. They are, also, 

 typically equal in length, as might be expected in an ^^^ with 

 scanty and evenly distributed yolk (Wilson, '92, Conklin, '97). 

 In Podarke the shorter furrow is always at the upper pole, but 

 in Thalassema we find every variation, even to its complete ob- 

 literation. As a rule the A and C blastomeres do not quite 

 touch at the upper pole, and through this space one of the 

 polar bodies has already slipped. All these four blastomeres 

 are exactly the same size and, although it is possible by means 

 of the polar furrow to distinguish the first cleavage plane from 

 the second (Conklin, '97), there is no way of telling the anterior 

 macromere, B, from the posterior, D (Treadwell, '01). As in 

 Podarke, the polar furrow remains constant until other means 

 of orientation appear. In Thalassema it is nearly at right 

 angles to the future sagittal plane as in Arenicola (Child, '00). 

 The various quartets and the first and second cleavage planes 

 bear the same relation to the future axes of the adult as in 

 Amphitrite, Arenicola and Podarke. An adequate discussion 

 of this point is given by Treadwell ('oi). 



The first quartet of micromeres ^ arise simultaneously and 

 are only slightly smaller than the *' macromeres " (Text- Fig. i, 

 C and D). At this stage there seems to be no tendency for one 

 quadrant to divide before the others. The small lenticular 

 cleavage space, which appeared as early as the two-cell stage, in- 

 creases rapidly in size as the development progresses. At the 

 transition from 8 to 16 cells the second quartet of micromeres 

 are formed by an equal leiotropic division of the macromeres 

 and the primary trochoblasts by a somewhat unequal leiotropic 



^The cleavages take place at intervals of about fifteen minutes, but, as in so 

 many other cases, are greatly accelerated by heat and retarded by cold. In the liv- 

 ing egg it is well-nigh impossible to follow the cleavages very far on account of the 

 great flattening out of the blastomeres during the resting periods. 



