132 Papers from the Marine Biological Laboratory at Tortugas. 



of the ciliated bands and the body parts of 28 parthenogenetic larvae 

 gave no half nuclei. The nuclei were either of normal size or were above 

 normal size. This leads to the conclusion that there must have been 

 many cases of monaster formation. 



Notwithstanding this complication, the comparison of the nuclear 

 size of purely parthenogenetic larvae with those of hybrids with displaced 

 heredity proved of value. Herbst succeeded in fitting the larvae that he 

 obtained to the types suggested by Boveri and in demonstrating the 

 actual participation of the sperm nucleus in the activities of fertilization 

 and of segmentation. 



Herbst first made a study of the nuclear size of parthenogenetic 

 larvae. He found: 



1. Diplothelykaryotic larvae. This was the most numerous form. 

 They may have arisen from a single monaster formation of a Monokaryon 

 (hemikaryon). 



2. Tetrathelykaryotic larvae. These may have arisen by two 

 monaster divisions; they have very large nuclei. 



3. Larvae with nuclei intermediate in size between (i) and (2). 

 These may have arisen, 



(a) From eggs of over-normal nuclei. These nuclei may have arisen 

 from one monaster division; they would be diplothelykaryotic. 



(b) From eggs of under-normal nuclei. These nuclei may have 

 arisen from two monaster divisions. They would be tetrathelykaryotic. 



(c) From eggs of normal nuclei. These nuclei may have arisen from 

 two monaster divisions, but in this monaster formation some of the 

 chromosomes must have remained undivided. 



Proof of the copulation of egg and sperm nuclei was drawn from 

 the occurrence of partially-thelykaryotic larvae. In such larvae (see 

 Herbst 1907, figs. 3 and 4) the skeleton of the anal arms is of the Sph<%- 

 r echinus type on one side and of the hybrid type on the other. This 

 condition may be due to the fact that the chromatic matter of the 

 maternal and paternal nuclei separated in the first division. A slight 

 difference between the two sides may be due to the influence of cytoplasm 

 given off by the male nucleus. Some indication of paternal influence 

 may be seen even in the maternal half. On the hybrid side of the pluteus, 

 large nuclei are found; on the maternal side small nuclei. 



A comparison of other larvae of the cultures was made with these 

 partially-thelykaryotic larvae, i.e., a comparison of nuclei was made. 

 Herbst reasoned that if he found larvae whose nuclei corresponded in 

 size with those of the hybrid side of the partially-thelykaryotic larvae, 

 he would be at liberty to conclude that in these cases a copulation of the 

 two sex nuclei had taken place. K he found, on the contrary, small 

 nuclei, he must conclude that in these the copulation had not taken 

 place and that he had hemikaryotic plutei. 



(i) Hybrids with large nuclei ; maternally directed heredity (Herbst's 

 figs. 6 and 7): These larvae were of a pronounced maternal type. Their 

 nuclei are large, like those of the hybrid half of partially-thelykaryotic 

 larvae. 



