1904.] NATURAL SaENCES OF PHILADELPHIA. 329 



vegetative pole (''macromeres") and from which the " niicromeres " 

 arise; for these latter the small letters a, b, c and d are used. Child 

 (1900) and Treadwell (1901) have been followed in giving coefficients to 

 the macromeres also, to indicate their generation, this being desirable 

 when dealing with an egg in which, after the first few cleavages, the 

 "macromeres" are large in name only, "Thus A, B, C, and D form 

 the four-cell stage. At their next division from A arises lA and la; 

 from B, IB and lb, etc.; lA then divides into 2A and 2a, while la 

 divides into la^ and la^" (Treadwell). 



Earlier Work on Opisthobranch Development. 



A rather large number of older investigators have worked upon 

 Nudibranch larval development. Grant (1827) described the veligers 

 of ^olis and Doris. In 1837 Sars discovered that the young of Tri- 

 ionia, Doris and ^olis possess a nautiloid shell ; additional researches 

 by the same investigator appeared in 1840 and 1845. Loven (1839) 

 described a number of Nudibranch larvae together with those of other 

 mollusks. Alder and Hancock's magnificent monograph upon the 

 British Nudibranchs appeared in 1845 and contains a good general 

 accovmt of the results thus far obtained upon the subject of Nudi- 

 branch embryology. Reid in 1846 published an interesting paper upon 

 the breeding habits of Doris, Goniodoris, Polycera, Dendronotus, Doto, 

 etC;, together with the constitution of the larvae. An account of the 

 embryology of Tcrgipes by Nordman appeared in the same year. An 

 extremely thorough account of the development of the Tectibranch 

 Adceon by Vogt also appeared in 1846. In 1848 Koren and Danielssen 

 described the early stages of a number of Nudibranchs from the Nor- 

 wegian coast. Schneider (1858) described the veliger of Phyllorhoe. 

 Keferstein and Ehlers (1861) gave an accoimt of some of the develop- 

 mental stages of jEoUs. 



The later investigations of Langerhans (1873), Lankester (1875), 

 Trinchese (1880-1-7), Lacaze-Duthiers and Pruot (1887), Rho (1888), 

 Mazzarelh (1892-3-5), Heymons (1893), Viguier (1898), Carazzi (1900), 

 Cluiart (1901), and other works upon Opisthobranch embryologj', 

 together with those of importance pertaining to the remaining mol- 

 luscan groups, Annelids and Platodes, will be considered during the 

 course of this paper. 



A good general account of spaM'uing habits of Nudibranchs is found 

 in Alder and Hancock's "Monograph of the British Nudibranchiate 

 Mollusca" (1845). 



