ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 629 



primary larva is converted into the Echinoderm appear to be 

 essentially the same in all cases ; all that happens in a more com- 

 plicated history being the fact that in the secondary larvae there is an 

 absorption of those larval parts which had themselves become 

 secondary. The secondary characters are not to be regarded as 

 having anything to do with the future organization of the Echinoderm, 

 but as adaptations, proper to the larval life, and disappearing with its 

 cessation. 



After giving a description of the mode of attachment of the eggs 

 by the female, and the mode of fertilization, the author states that the 

 two first cleavage spheres are almost equal in size ; differences, how- 

 ever, soon become apparent since one divides before the other. The 

 results of cleavage are, not a solid morula, but a blastosphere with a 

 unilaminate wall; the gastrula is formed by invagination. The absence 

 of a morula would appear to be a constant phenomenon among the 

 Echinodermata, and no other mode of formation of the gastrula than 

 that by invagination has ever yet been observed. The mesoderm 

 would appear to be generally derived from the endoderm, the cells 

 wandering in the fluid of the blastoccel : some share, however, is 

 taken by the ectoderm, and it may be perhaps justifiably supposed 

 that this mesoderm presents indications of a bilateral symmetry. 



The gastrula-enteron is composed of two chief portions, a short 

 cylindrical entrance, and a spacious vesicular terminal part ; from 

 the latter we have developed the enteroccal, the first signs of which 

 are the appearance of an out-growing process on either side, the 

 enteroccelic pouches. All this region is to be considered as being 

 essentially nothing more than a vesicular enlargement of the blind 

 end of the primitive intestine. Up to and at this time the structure 

 of the larva is still in all points bilaterally symmetrical ; but this 

 symmetry soon begins to disappear, the enterocoelic pouches are no 

 longer of the same size, owing to the much greater development of 

 the left one. The external form also becomes altered, and on the 

 fourth day there appears at the centre of the anterior side a de- 

 pression of the ectoderm, which is the rudiment of the mouth and 

 stomodaeum of the larva. Towards the end of the fourth, or on the 

 fifth day, the enteroccel becomes completely shut off from the larval 

 enteron and the larval stornodteum opens into this latter. During the 

 fifth day the rudiment of the water-vascular system becomes developed 

 from the left enteroccelic pouch, and this outgrowth may be con- 

 veniently spoken of as the hydroccel ; connected with it are the rudi- 

 ments of the five primary vessels of the water-vascular system, 

 which first appear as five slight outgrowths. Contemporaneously 

 with the development of the hydroccel we have the formation of the 

 dorsal pore of the larva, which is due to an invagination of the 

 ectoderm, which becomes connected with the left enterocoelic pouch. 

 With the exception of the observations of Kowalevsky on Psolinus 

 hrevis, the evidence of all embryologists would lead us to suppose 

 that the history given of the hydroccel of Asternia is generally true 

 of all Echinoderms. The author is inclined to look upon the dorsal 

 pore, in its primary relations, as being a pore which ltd into the 



