ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
335 
completed the mesoderm-gonads lie below the nerve-plate ; posteriorly 
they take part in the formation of the chorda dorsalis. They give rise 
to mesodermal elements, but remain themselves true endodermal cells. 
The gastric mesoderm is developed bilaterally, and becomes divided 
into the somatic and caudal mesoderms. The latter persists as a solid 
rudiment and becomes converted into the muscular layer of the tail, 
while the former is gradually lost in the mesenchym. 
The pregastric mesoderm arises much later from the cells of the 
pregastric endoderm that lie in front of the intestine ; it becomes gra- 
dually converted into mesenchym-cells, which agree in all points with 
those of the gastric mesoderm. The somatic and pregastric mesoderm 
unite, finally, into a common tissue, the body-mesenchym. It is to be 
specially noted that no signs of segmentation are to be seen in the 
gastric mesoderm, and in the caudal there is no indication whatever of 
a cavity comparable to a myocoelom. 
The history of development seems to show that neither Amphioxus 
nor the Ascidians are derived from ancestors which can be supposed to 
have been Enterocoelia. 
The development of the medullary tube of Distaplia is not so simple 
as might have been supposed. Three parts have to be distinguished, 
two of which belong to the pseudembolic, and one to the epibolic region. 
Posteriorly, in the region of the future tail, elements of the nerve-plate 
enter into the formation of the medullary tube, but more anteriorly 
ectodermal cells form covering cells for the medullary tube, and only 
much later become differentiated into nerve-cells. From the epibolic 
portion of the embryo the medullary tube obtains an increase in material 
which forms the anterior wall of the future sensory vesicle, and is not 
developed in the ordinary way ; the ectodermal cells multiply and give 
rise to a uni- or bilaminate rudiment, which is overgrown from the sides 
by ectodermal cells. 
Amphioxus and the Ascidians are distinguished from Vertebrates by 
the fact that in the last the medullary plate closes from before back- 
wards, and not in the opposite direction as in them. It is not yet quite 
clear which is the more primitive of these two modes of development. 
0. Bryozoa. 
British Species of Crisia.* —Mr. S. F. Harmer has come to the 
conclusion that the British fauna includes more species of the genus 
Crisia than are generally recognized ; this result is based on a com- 
parison of the ovicells, and especially of their apertures. He finds that 
the essential characters of the ovicells are extremely constant, in spite 
of the occurrence of variations of no inconsiderable magnitude in other 
parts of the colony. 
The author thinks that it is necessary to pay careful attention both 
to the number of zooecia in the individual internodes and to the character 
of the branching, and he has devised a method of graphic representation 
to show these points in each species. 
C. denticulata Lamk., C. eburnea Linn., C. aculeata Hassall, C. ramosa 
sp. n. (from Plymouth) are diagnosed. 
* Quart. Journ. Micr. Sci., xxxii. (1891) pp. 127-81 (1 pi.). 
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