138 THE VOYAGE OF H.M.S. CHALLENGER. 



to study the formation of the germ-layers, nor the modifications which they undergo 

 durino; development. Whether all the cells of the embryo in this stage are derived 

 exclusively from the blastoderm, or whether they are also partly due to the deutoplasm is 

 a question which it is impossible to answer from the section before me. Dorsally the 

 greater part of the embryo is- covered by a single row of flatten.ed cells (the original 

 blastoderm cells), ventrally a plate is clearly distinguished much thicker than the blasto- 

 derm, and doubtless formed of cells more than one row deep. Unfortunately, however, 

 the limits of these cells were quite gone ; I therefore could not distinguish either their 

 number or arrangement, Ixit I believe the evidence is great that in the inner layer of this 

 plate the original mesoblast is to be seen. In this stage rudiments of the appendages 

 are distinctly formed ; and I consider it a very characteristic feature in the development 

 of the Pycnogonids, that the food-yolk penetrates into these appendages. In the section 

 here figiu^ed, however, that part of the food-yolk which penetrates the leg, is not in direct 

 connection with the central food-yolk mass ; but this is caused Ijy the circumstance, that 

 the section does not pass exactly vertically through the embryo, but goes a little obliquely 

 from above backwards to the ventral side. 



The blastoderm shows to a considerable extent in the stage I have figured a double cell- 

 layer dorsally in the middle, and even a small lumen is observed between these two. 

 Small cells or nuclei seem to be present in this lumen, and the whole arrangement made 

 me think it ])Ossil)le that I had an early stage in the development of the heart before 

 me. The broad and flattened condition of the heart in the adult animal of Nymphon 

 is not opposed to this suggestion ; yet it is difficult to understand why a heart should be 

 developed l)efore there seems to be any question of an intestinal tract. 



About the same stage is also figured in figs. 9 and 10. At the ventral side the 

 first pair of appendages (the foot-jaws), three pairs of legs, between the foot-jaws 

 the proboscis, and the caudal protuberance, are easily distinguished. The second 

 and third pair of cephalic appendages show in this species a remarkal^le retardation 

 in their appearance, visible in the stage in which the first and second pair of true 

 legs are already two-jointed and bent inwards so as to meet in the middle of the 

 ventral surface, and in which the thii-d pair is longer, yet bent inwards and forwards. 

 In this same stage the third cephalic appendage is not yet distinguishable, and the 

 second pa:ir only shows a small protuberance at the base of the foot-jaws. An equatorial 

 section of an embryo in this same stage is figured in fig. 11. Between the foot-jaws («) 

 and the first true leg (h) two small protuberances are distinguished, the first of which (c) 

 is larger than the second {d), which in this stage is observed only interiorly. The 

 section- is also remarkable for the distinctness with which the nerve ganglia are seen. 



There is good reason to consider this arrangement characteristic for the species 

 Nymphon brevicaudatum, Miers. Other species of Nymphon, of course, may show 

 the same ; so far as I could ascertain it is not the rule, for neither Nymphon 



