Artificial Parthenogenesis in Thalassema Mellita 12 7 



The establishment of the second spindles in quite a normal man- 

 ner may be seen in Figs. 31, 32 and ^1^^ and in the latter the cen- 

 trosomes of one of the cells are clearly double. This bipartite 

 condition of the centrosomes has been observed repeatedly during 

 mitosis at all stages of the development (Figs. 35, 36, 40), but I 

 have not been able to demonstrate their persistence through the 

 resting period from one generation of cells to the next. After the 

 nuclei return to the resting condition, every trace of asters and 

 centrosomes disappears, and their presence is not revealed again 

 until the early prophase of the next mitosis. I am inclined to be- 

 lieve that, although the centrosomes may divide and diasters arise 

 at the close of a mitosis, these totally disappear, and that the asters 

 for the next cleavage are new formations brought into evidence 

 when the activities of the cell which bring about division are 

 renewed. 



Fig. 34 is drawn from a four-cell stage during the resting period, 

 and Fig. 35 from an eight-cell stage, showing four of the cells in 

 mitosis; the early formation of the cleavage cavity is seen in the 

 latter figure. Later stages in the formation of the blastula are 

 illustrated in Figs. 36 to 40, in each of which one or more cells are 

 taken in mitosis. Fig. 39 is probably somewhat abnormal, as it 

 shows an unusually large blastocoel, a not infrequent condition. 



In Fig. 36, the section has passed through the equatorial plate 

 in the cell indicated at a, and here the twelve chromosomes can be 

 easily counted. As will be emphasized beyond, the reduced num- 

 ber of chromosomes has been repeatedly determined in cells of 

 blastulae and gastrulae (Fig. 41, a) and the evidence is, therefore, 

 perfectly clear that, except under certain abnormal conditions 

 which will be referred to, a restoration of the normal number of 

 chromosomes does not occur. 



4 Gastrulation and the Formation of the Trochophore 



Gastrulation, which according to Torrey ('03) takes place 

 about seven hours after fertilization, is considerably delayed in 

 parthenogenetic development and occurs at from eight to twelve 

 hours after exposure to acid solutions. The process is of the 

 embolic type, and the insinking of the entoblastic plate can be 



