98 



PSYCHE. 



[June 1S91. 



we take into consideration the great extent 

 of the median invagination in Musca and the 

 small size of the blastopore in certain Or- 

 thoptera where no lateral gastrulation has 

 been observed. 



One may venture to object to some of the 

 new terms of which Prof. Graber has been 

 rather prodigal in his latest papers. The 

 time-honored term "blastoderm" is easily 

 understood and it is difficult to see why it 

 should be relegated to the biological attic for 

 effete nomenclature to make room for an only 

 remotely suggestive term like "cycloblast." 

 Prof. Graber now dubs the yolk-cells "cen- 

 troblasts" notwithstanding the termination 

 "blast" is properly applied only to tissues of 

 a germinal or formative character and not to 

 elements which, like the yolk-cells, degener- 

 ate and take no part in building up the insect. 

 It would be wiser to suspend the use of terms 

 like "ptychoblast" till we possess a better 

 knowledge of "arthropod" germ-layers, as we 

 have still a great deal to learn on this sub- 

 ject. In the meantime "mesentoderm" is 

 quite clear and will answer all purposes. 

 "Entomyoderm" and "ectomyoderm" are 

 scarcely to be regarded as improvements on 

 the old terms, "splanchnopleure" and "soma- 

 topleure." We supposed that Prof. Graber's 

 terminological fecundity was exhausted 

 when he gave us the sesquipedalian, "ec- 

 toptygmatorhegmagenous ptychonotogony.' 

 But these are small blemishes in a work 

 which will rank among the more important 

 contributions to insect embryology. 



The chief value of Prof. Graber's paper 

 cannot be said to lie in a furthering of what 

 we must regard as one of the chief aims of the 

 study of insect development, viz: a know 

 edge of the mutual phylogenetic relations of 

 the existing orders of insects (often separated 

 by wide gaps, towards the bridging of which 

 comparative anatomy and paleontology have 

 contributed only a little), and a knowledge 

 of the relations of insects, as a, class, to other 

 arthropod groups and to their remote ances. 

 tors, the annelids. In our estimation Prof. 



Graber's work is chiefly valuable as showing 

 to what an extent the embryonic develop- 

 ment of a calyptrate muscid has been de- 

 flected from the ancestral path — in other 

 words, it is an admirable picture of one of 

 the "short-cuts" in insect development. 



It will be remembered that the egg of the 

 fly hurries through its whole development 

 in about 24 hours, that it is provided with a 

 relatively small quantity of yolk, and that 

 the characters of the secondarily developed 

 and degraded maggot have been reflected 

 back into embryonic life. That this reflection 

 has materially altered the original ontogeny 

 as displayed by older forms such as the Or- 

 thoptera and Hemiptera, is evident from the 

 fact that the embryo no longer exhibits the 

 typical cephalic and thoracic appendages, to 

 say nothing of the abdominal appendages 

 (embryos of bees, beetles with apod larvae 

 and fleas still develop thoracic legs!). More- 

 over the ammion and serosa are rudimen- 

 tary in Musca; the head has become pro- 

 foundly and strangely modified and the 

 mesodermic layer no longer exhibits the typi- 

 cal paired coelomic cavities. It is, therefore, 

 obvious that conclusions drawn from the 

 embryogeny of a muscid cannot without 

 extreme caution be extended to cover other 

 insects. It is further evident that to ascer- 

 tain the exact path of development in an 

 insect that develops so rapidly a very great 

 number of eggs must be examined to insure 

 certainty in regard to the different steps in 

 the hurried sequence of tissue-changes. 

 Granting the correctness of Prof. Graber's 

 observation that the proctodaeum is of mes. 

 entodermic origin, we may perhaps account 

 for this condition on the supposition that 

 whereas in other insects of slow development 

 the hind gut is not formed till after the clos- 

 use of the posterior end of the blastopore, 

 in Musca the processes of growth succeed 

 one another so rapidly that the blastopore 

 does not have time to close before the hind- 

 gut is found. Thus the apparently mesento- 

 dermic character of the hind gut would 



