SUMMARY OF CURRENT RESEARCHES. 
181 
Tlie formation of the functional allantoic 'placenta is brought to pass 
through the gradual degeneration and resorption of the enlarged cho- 
rionic ectoderm cells over the placental area proper. The allantoic 
capillaries can now directly reach the vascular surface of the allantoic 
placental syncytium, with which they become intimately attached. The 
foetal and maternal blood-streams are now separated only by their thin 
endothelial walls, and perhaps by a thin layer of syncytial protoplasm. 
III. Parturition. There is no decidua at birth, but the vesicular 
portion of the allantois remains persistently attached to the placental 
syncytium, and is gradually absorbed in situ along with the latter by 
the maternal leucocytes. 
The foetus, while still connected with the placental area by the 
lengthened allantoic stalk, passes out, not by the lateral vaginal canals, 
but by breaking through a median track leading backwards from a 
posterior common portion of the two uteri. 
Early Development of Amphioxus.* — Prof. E. W. MacBride has 
re-investigated this, and draws the following conclusions : — (1) The 
primitive gut or archenteron is formed in Amphioxus by a typical process 
of embolic invagination, the endoderm being at first not sharply marked 
off from the ectoderm. The blastopore is at first posterior, but subse- 
quently becomes dorsal by the preponderant growth of the ventral 
(2) The mesoderm originates in Amphioxus as a series of true gut- 
pouches, viz. one anterior unpaired pouch and two pairs of lateral 
pouches. The first divides to form the two head-cavities ; the anterior 
pair give rise to the first pair of myotomes, and, in addition, to two long 
canals extending back ventrally ; the posterior pair, gradually separated 
from the gut, form the series of myotomes. 
(3) Hatschek’s nephridium is the persistent connection of the left of 
the pair of collar-pouches with the gut. (4) The metapleural “ lymph- 
canals ” found in the atrial folds are the persistent ventro-lateral exten- 
sions of the “ collar- pouches.” 
He also concludes that all attempts to explain the formation of the 
nervous system of Vertebrates by the coalescence of the two halves of 
a nervous ring lying in the lips of a long slit-like blastopore must be 
given up ; and that the theory of the descent of the Vertebrates from 
a form somewhat like Balanoglossus receives strong support from the 
early developmental history of Amphioxus. 
Development of Atrial Chamber in Amphioxus.f — Prof. E. Ray 
Lankester makes some corrections on MacBride’s paper in regard to the 
atrial chamber. Instead of confirming Kowalevsky’s conclusions, as 
MacBride says, Lankester and Willey showed that there are no atrial 
folds. What Kowalevsky mistook for “ atrial folds ” are really the 
metapleura. These do not grow round and meet in the middle line, but 
a very small in-sinking is formed between them, and is covered in by a 
minute horizontal growdh right and left (the subatrial ridges or folds), 
their union resulting in the formation of what is, at first, a very narrow 
“ subatrial floor ” lying between the two upstanding metapleura. 
* Quart. Journ. Micr. Sci., xl. (1898) pp. 589-612 (3 pis,). 1 
t Tom. cit., pp. 6t7-50. 
1898 
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