296 EGBERT TRACY JACKSON ON THE 



in the 03'ster, tlic gastrula invagination commencing before the formation of a hollow 

 sphere. We have, therefore, no stage comparable to our mesembryo stage, PI. xxni, 

 fif's. 1, la, its absence being explicable as a case of extreme concentration of develop- 

 ment in the oyster. 



In the latei- segmentation stages. Brooks' figs. 22-28, also our fig. 14, we find that 

 those cells which are of ectodermic and entodermic nature are clearly defined. The en- 

 toderm consists of a single maci'omeric cell surrounded in an increasing degree by mi- 

 cromeric cells up to the period shown by Brooks' fig. 26. The single jnacromere then 

 divides into two, Brooks' fig. 28, then into several cells, his fig. 30. Coincident with 

 the splitting up of the entodermic cells tliei'e is, according to both Brooks' figures and 

 our fig. 16, a flattening of the embryo from above downwards, and a pushing in of the 

 entodermic cells, which commencing as a slight depression, Brocdvs' fig. 31, our fig. 16, 

 soon forms a deep invagination, the gastruhi cavity; see Bi ooks' figs. 32-35 and our 



From this point on thei'c are some important diflfereuces in the observations of Pi'o- 

 fessor Brooks and Dr. Ilorst. As Horst's observations were made on emljryos of 

 O. edulis, which are much larger than those of O. virc/hiiaun, and as they are more in 

 accordance with observations made on the development of other Pelecypoda and mol- 

 lusca I have for the most ])art followed them where difi*ei'ences exist. 



In our classification of the stages of growth, the metembryo mollusc represents the 

 gastrula stage, PI. xxiii, fig. 2, being a period in which two well-formed and an incijHent 

 thii-d layer exist. Professor Brooks' figures 32-35 represent difierent aspects of the gas- 

 trula oyster according to his observations. His fig. 33, an optical section, shows an em- 

 bryo with a primitive gastrula cavity and body cavity. It also has a velum slightly devel- 

 oped. These figures answer well the requirements of the metembryo stage and may be 

 said to represent tliat period in the development of Ontrea virf/iiiiana. 



Dr. Horst, considering the stages (our figs. 16-17) which are referal)le to the gastrula 

 period, says, " there can however be no question of a genuine invagination because there 

 is no cavity of segmentation; we have here, so to speak, an intermediate form between 

 an embolic gastrula and an epibolic gastrula.^" He notes the remarkable fact that coinci- 

 dent with the gasti'ula invagination at the vegetative pole, there is also a very distinct 

 depression at the other pole, a little below the doi-sal side, oui* fig. 17sk. This second in- 

 vagination by further development ("our figs. 18-19) he shows is the beginning of the 

 preconchylian invagination. 



The gastrula invagination, accoi'ding to his figures, begins before the preconchylian in- 

 vagination; but the latter exists before the completion of the gastrula invagination. 

 Horst's figiu'es do not show any trace of a velum at this period, which is somewhat pecu- 

 liar, as he represents the oyster as so much more advanced in its molluscan characters than 

 does Brooks, who figures the velum at the gastrula period. The early absence of the velum 

 in O. echdis may be connected with the brooding habit of that species, for in Pelecypoda, 



' Accordiiij; to Hyatt (Lai-val Theory of tlie Origin of a process imicli shorter than is usual in embryos of tlie 



Tissue, p. 156), in the formation of epibolic gastrnhe embolic type. He accounts for the epibolic gastrula by 



the jirowth of the ectoblast cells is so rapid that the endo- heredity and extreme concentration of development, 

 blast cells become enclosed, and the gastrula is formed by 



