21)6 ROBKRT TRACY JACKSON ON THE 



ill llie 03'ster, the g-astrula invagiiifition commencing before the formation of a hollow 

 sphere. We have, theiefore, no stage comparable to our mesenibryo stage, PI. xxiii, 

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

 ment in the oyster. 



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

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

 toderm consists of a single macromeric cell surrounded in an inci'easing degree by mi- 

 cromeric cells up to the jx^iod shown by Brooks' fig. 26. The single macromere then 

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

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

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

 entodermic colls, which commencing as a slight depression. Brooks' fig. 31, our fig. 16, 

 soon forms a deep invagination, tlie gastrula cavity; see Biooks' figs. 32-35 and our 



fig- 17. 



From this point on there are some impoi-tant differences in the observations of Pro- 

 fessor Brooks and Dr. Horst. As llorst's observations were made on embryos of 

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

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

 Insca I have for the most part followed them where diffei-ences exist. 



In our classification of the stages of gi-owth, the metembryo mollusc represents the 

 gastrula stage, Pl.xxui, fig. 2, being a period in which two well-formed and an incipient 

 third layer exist. Professor Brooks' figures 32-35 represent different 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 meteini)iyo stage and may be 

 said to represent that j^eriod in the development of Ostrea vtrginiana. 



Dr. Horst, considei-ing the stages (our figs. 16-17) which are referable 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 i'act that coinci- 

 dent with the gastrula invagination at the vegetative pole, there is also a very distinct 

 depression at the other pole, a little below the doi-sal side, our fig. 17.^^-. This second in- 

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

 preconch^dian invagination. 



The gastrula invagination, according to his figures, begins before the preconchylian in- 

 vagination; but the latter exists before the completion of the gasti-ula invagination. 

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

 liar, as he represents the oyster as so much moi'e advanced in its molluscan characters than 

 does Brooks, who figures the velum at the gastrula pei'iod. The early absence of the velum 

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



' According to Hyatt (Larval Tlieory of the Orisriii of a process much shorter than is usual in embryos of the 



Tissue, p. 156), in the fonnatiou of epibolic gastrula; embolic type. He accounts for the epibolic liastrula by 



the growth 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 l>y 



