STEUCTDRE OF THE LARVA OF SPONGILLA LACUSTRIS. 435 



the nuclear spiudle liave not yet appeared^ or are indistinctly 

 seen. It seems that they are in the process of development 

 (fig. 42, 2 a, 2 b). In the next stage the chromatin assumes 

 the form of a well-defined equatorial plate. The individual 

 granules (or chromosomes), which were difficult to identify 

 in the previous stage, are much more distinct, being appa- 

 rently twelve in number. Each one of these chromatin 

 granules seems to divide, so that a double plate of granules 

 appears in the median plane of the spiudle (fig. 43, 3 a — 3 e). 

 In this, the metaphase stage, the spindle threads are dis- 

 tinctly seen, but the poles of the spiudle seem to be em- 

 bedded in a cloudy, ill-defined area of the cytoplasm. The 

 nuclear spindle has evidently arisen inside the nucleus, for the 

 nuclear membrane in many cases still exists. Though I 

 have often suspected the presence of a centrosome iu 

 the cloudy area of the cytoplasm, I have not yet been 

 able to demonstrate its existence with certainty. The two 

 small bodies in fig. 43, 4 a, have probably no connection 

 of any kind with the centrosome, supposing such a body 

 to exist. The chromosomes after dividing at the centre pass 

 in two groups towards the poles of the spindle (fig. 43, 4 a, 

 4 b). When they have reached the poles of the spindle^ they 

 are arranged at first more or less in the form of a ring in 

 which the individual granules are distinct (fig. 43, 5 a and 5 b). 

 During the passage of the chromosomes from the centre to 

 the opposite ends of the spindle the spindle threads are lost 

 sight of, but after the passage they again become distinct. 

 Concurrently with the changes represented in fig. 43, 4 a — 4 c, 

 the nuclear membrane disappears. Whether the threads 

 visible in fig. 43, 5 a and 5 b, are the same as those seen in 

 3 a — 3 e, or are developed from the nuclear membrane which 

 has disappeared, I am uncertain, but incline towards the latter 

 view. The chromosomes after reaching the poles of the 

 spindles soon lose their individuality, to all appearance at 

 least, and become united together so as to form a kind of cap 

 of chromatin, which at first gives some indications that it has 

 been formed by the coming together of several bodies. From 



