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A Preliminary Note on the Embryology of the 

 Pycnogonids. By T. H. Morgan. 



The position of tlie Sea Spiders, amongst tlie Articulates, and tlie specu- 

 lations as to the relationship of this to other orders, have been almost 

 entirely based on the anatomy of the adult animal. Kroyer, in 1840, 

 described some embryos, and Dohrn and Hoek have carried this a step 

 further. Owing to the extreme difficulties of technique, nothing is known 

 of the internal changes that take place during development. 



A study of the germ layers and their subsequent differentiation into 

 organs ought to throw, I thought, some light on the phylogeny of these 

 most interesting animals. Material for work was collected during the 

 summer of 1889 at Wood's Holl, Mass., and I am under very great obliga- 

 tions to Professor McDonald for the opportunity to collect and study at the 

 laboratory of the Fish Comgiission Station. 

 I Three species of Pycnogonids are found amongst the sea-weeds and 

 hydroids about Wood's Holl, viz. : Pho.xichilidium maxillare, Tanystylum 

 orbiculare, and Pallene empusa. During July and August these carry eggs. 



The first two genera have a small free-swimming, six-legged larval stage. 

 In Pallene the eggs are much larger, resulting in abbreviated development, 

 and the young leave the cluster of eggs, carried by the male, in a practi- 

 cally adult condition. 



In Phoxichilidium and Tanystylum the egg undergoes a regular segmen- 

 tation into 2, 4, 8, 16 ; all the segments being equal. After one or two 

 more divisions a condition is reached as shown by Fig. 6. Here the seg- 

 ments have the form of pyramids, with the apices together at the centre of 

 the egg. A nucleus is at the outer part of each pyramid. The egg con- 

 tinues to divide, the pyramids becoming smaller, and then each pyramid 

 divides into an outer and an inner part, each part with a single nucleus. See 

 Fig. 7. Here we have a most perfect delamination, resulting in an outer 

 circle of ectodermal cells and an inner mass of cells. Both inner and outer 

 cells continue to divide. Many of the inner cells now break down, as is 

 seen in the endoderm of many Coelenterate planulae, and the cells of the 

 outer circle become smaller ; the line of demarcation between inner and 

 outer cells remains sharp and distinct. It is very difiicult to follow out the 

 fate of these two cell masses. Many of the inner cells seem to form a yolk- 

 like substance, with a few scattered nuclei, while the outer cells form 

 undoubtedly the ectoderm of the adult. I believe the endoderm to be 

 formed from some of the nuclei of the inner cells, but I cannot be entirely 

 certain of this, nor have I any observations as to the origin of the mesoderm. 



In Pallene, on account of the much larger and more manageable eggs, I 

 have been able to carry out in much more detail the origin and fate of the 

 germ layers. The eggs measure .25 mm. in diameter, and have 125 times 

 the volume of the preceding species. The segmentation is quite interesting, 

 and I had the good fortune to be able to follow it quite far along in the 

 living egg. 



