24 T. H. MORGAN. 



segments round off to form the second furrow, and this seems 

 to be the stage which Hoek has described as the first segmenta- 

 tion. Nor does it seem probable that the differences of our 

 observations are due to their having been made on different 

 species, for in each case the egg is approximately the same size. 



The changes which take place in the formation of the endo- 

 derm of Pallene may, I believe, be referred to the simpler 

 changes of Phoxichilidium, etc., and furnish in this respect an 

 excellent basis for further comparison with other forms having 

 much yolk present. In Phoxichilidium the pyramidal segments 

 divide into an outer and an inner cell, while in Pallene the 

 nuclei alone divide; and although delamination is still multi- 

 polar, it is not synchronous over the egg. A further comparison 

 of the mesenteron I am unable to give, owing to lack of com- 

 plete observations of the changes of Phoxichilidium. In both 

 types the oesophagus invaginates with a triangular lumen, and 

 in each the proctodeum forms later in development. 



The few observations I have made on the ventral organs of 

 Tanystylum leave no doubt that it is the same structure as in 

 Pallene. Prof. Sedgwick has described in Peripatus paired 

 ventral organs which correspond in number and position with 

 the pairs of ventral ganglia. Comparing Fig. 21, Plate II, with 

 his figure for the ventral organs of the jaws of Peripatus (Studies 

 Morph. Lab., Camb., Vol. IV, Pt. I, Plate 10, Fig. 4), a striking 

 similarity is seen, and, in addition to this, he says in his account 

 of the ventral organs that they are slightly invaginated from the 

 surface. Whether these structures are in any way related it is 

 impossible to say, but it is worth while to call attention to the 

 close similarity both in position and structure between these 

 organs in the two groups. 



A comparison of the appendages and their order of develop- 

 ment is of interest. Prof. Dohrn has most carefully and fully 

 worked out the transformations of the larval forms, and through 

 his skill we have a very thorough knowledge of the transforma- 

 tion of the embryo. According to his account the six-legged 

 larvae of the Pycnogonids with the Pantopod-development pass 

 into the adult condition by the body elongating behind the last 

 pair of appendages and the walking legs appearing from before 

 backwards, in much the same way as Prof. Claus believes the 



