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74 Haeckel’s Gastrea Theory. [ February, 
importance. It is by palingenesis and cenogenesis, the terms he 
applies to primary and secondary embryonic phenomena, that he 
accounts for the divergence observed in the earlier embryonic 
stages. Whether we agree with Haeckel or not, his paper can- 
not fail to be most suggestive, as this is the first attempt to tabu- 
late the early embryonic stages of the egg in the different classes 
of the animal kingdom, with a view to account for their differ- 
ence on the theory of natural selection ; the more interesting, com- 
ing as it does from the investigator who first tested the theory of 
descent by the monographie study of a great group. It is not 
our purpose to describe the many subordinate phenomena, either 
of palingenesis or of cenogenesis, quoted by Haeckel ; we merely 
wish to call attention to the dangerous path he treads when he 
explains anomalies as falsifications of the record in either time or 
space. When we have to resort to such devices, no explanation 
at all is fully as satisfactory. 
Armed with this new instrument of investigation, Haeckel 
carefully compares the different modes of segmentation resulting 
in the gastrula, to which he had already alluded in his Anthro- 
pogenie. He then takes up the same subject for the several 
classes of the animal kingdom, and treats it with his usual inge- 
nuity, and closes with the phylogenetic interpretation to be as- 
signed to the early stages of embryonic development. Of these 
he recognizes five as of primary importance: the ““monerula,”’ or 
the first stage of metazoan development ; the second stage, repre- 
senting the egg as commonly understood, which he calls the « cy- 
tula;” the third, the « morula,” or mulberry stage; the fourth, 
the “planæa ” (formerly known as “ planula,” though very dif- 
ferent stages were often spoken of under that name); and the 
fifth, the “ gastrula.” 
This paper is accompanied by two plates of diagrammatic 
sketches copied from various authors, representing the segmenta- 
tion and gastrula of various invertebrates and vertebrates. 
Haeckel gives in addition original figures of the same stages in a 
crustacean, an annelid, a mollusk, and a bony fish, It is a great 
pity that such a skillful draughtsman should give such untrust- 
worthy figures to illustrate so fundamental a theory, and quietly 
fall back upon the righteousness of his cause. His figure of a 
fish embryo has no value as a copy of nature; it is a diagram 
simply. Such an embryo may exist, but the distrust naturally 
felt of such fictitious illustrations, by all who are familiar with a 
portion of his subject, naturally extends in the first place to all 
his figures and lastly to his whole theory. 
