136 Papers from the Marine Biological Laboratory at Tortugas. 



MOIRA. 



The Moira pluteus (plate i, figs. 5, 6, and 7) in the older stages is 

 characterized by the possession of a posterior unpaired spine whose 

 method of origin is shown in plate i, fig. 5. The anal arm rods are 

 fenestrated. 



TOXOPNEUSTES. 



The Toxopneustes pluteus (plate i, figs, i and 2) is more slender 

 in form than the others considered. It differs from them further in 

 that the skeletal rods of the anal arms are single, slender rods whose 

 surface is roughened by the presence of thorn-like projections. The 

 dorsal and ventral body skeletons are not connected posteriorly. A 

 detailed consideration of these plutei of various ages may be found in 

 my paper on variation in Toxopneustes plutei (Tennent 1910). 



THE CROSSES. 

 Arbacia: 



Two crosses with the Arbacia egg were niade, one with Mellita and 

 one with Moira. A fertilization membrane was formed. In both in- 

 stances cleavage took place about 40 minutes after fertilization, the time 

 of beginning of cleavage thus not being changed by the use of foreign 

 sperm. The Arbacia egg is small and deeply pigmented, and is not 

 adapted for study of the nuclear activities in the living conditions. 

 I have shown elsewhere (1908) that the preserved egg is especially 

 favorable for the study of chromosomes in cross-fertilized eggs. 



Arbacia? X MellitaJ": 



The greater number of plutei obtained from the ^4 rbacia ? X Mel- 

 litad" cross (plate 2, fig. 22) could be referred neither to the maternal nor 

 to the paternal type. In most cases the skeleton of the anal arms con- 

 sisted of from two to four unconnected or irregularly connected rods. 

 No plutei of a pronounced paternal type were found. The posterior 

 basket was undeveloped. About 2 per cent were of the maternal type 

 shown in plate 2, fig. 22, although here the hybrid characters of the 

 larvae are at once evident. The plutei are not intermediate in type, but 

 form a series. The body is in general of the Arbacia type. 



Arbacia 9 X MoiracJ': 



The Arbacia^- X Moira cj' cross was readily made and was very 

 successful for segmentation, but was of little use in the older stages. 

 The eggs were allowed to stand for 7 hours and were then fertilized with 

 active Moira sperm. The hybrids were irregularly intermediate in 

 character. No trace of a posterior unpaired spine could be seen. The 

 hybrids inclined somewhat to the maternal form, yet showed distinctly 

 their hybrid origin. 



Toxopneustes: 



It will be remembered that the Toxopneustes plutei are distinctive 

 in form, in that they alone have single rods in the anal arms. The plutei 

 of all other forms worked with have fenestrated rods. Anv cross with 



