240 EMBRYOLOGY. 



EMBRYOLOGY. 



Ill the report of the Solenogastres of the North Pacific a species of neome- 

 nian, Halomenia gravida, was described that carried in the spaces between the 

 branchial folds of the cloacal chamber twenty-five embryos in various stages 

 of development. As the single specimen was sectioned before these were dis- 

 covered it is ob\aous that the following account of the earlj^ g^o^\^;h of this 

 species has been based solely on sections and reconstructions. It may be said, 

 however, that unusual care has been exercised in their study, and while certain 

 details, to be noted later, are doubtless faulty, the broader features are fairly 

 clear and intelligible. 



In the earhest stage two distinct nuclei are present (Plate 13, fig. 5) together 

 \\ath four other bodies that may be nuclei, though appearances (Plate 13, fig. 

 10), suggest that more probably they are unusually large yolk granules super- 

 ficially coated with a dense layer of protoplasm or some glandular secretion. 

 The egg measures at this time 0.32 mm., assuming that a slight elongation is 

 due to the pressure of the neighboring branchial folds. A single polar body 

 (Plate 14, fig. 10) is attached, and it is a peculiar fact that in every case where 

 these cells have been seen there is never more than one. A delicate membrane, 

 cuticular in appearance, surrounds the egg, and where it has remained undis- 

 turbed it is closely adherent to the yolk granules beneath. At other points on 

 the surface it niaj^ be thrown into folds, but as these lack regularity and any 

 signs whatever of nuclei, it is probably a viteUine membrane and not a chorion, 

 though this last named structure occurs normally in the Chitons and several 

 Solenogastres. Furthermore in developing ova within the reproductive gland of 

 Halomenia there are no traces, even in early stages, of a chorion though the 

 cuticular membrane just described is well defined. Immediately within the 

 membrane, and adjacent to the polar body, a clearly defined nucleus exists 

 closely surrounded by yolk with the exception of an excentrically placed archi- 

 plasm mass. It is probably the female pronucleus, while the remaining one of 

 similar appearance is the sperm nucleus. 



In the only other early stage two undoubted nuclei hold the same relative 

 positions, and again there are four bodies that as before may be either nuclei 

 or yolk granules. It is worthy of note that one or two similar bodies may occur 

 in fairly advanced embryos, and on the other hand none of these bodies has ever 

 been detected in mature ova within the gonad. Furthermore in these early 

 stages no cleavage planes have been noted. 



