2 CARL G. HARTMAN 
jARRHeyprimmbiviexento dena... savrdcie cise ac c/s ore SIO OIC en eee 65 
ky Furtherierowth-of the blastocyst... i. 5i\ssetoa: «+ cece setae erie pees 65 
1. The end of entoderm formation and the spreading of the entoderm.... 68 
m. The maximum. attenuation of the trophoblast......................... 70 
n. The cause of the spreading of the entoderm.............:....-2:ss-«¢+ 71 
o. The changing shape and position of the blastocyst.................... 71 
De Some, abnormal: egg: 72.251 ee a eee wee ee er nee ele) oof cog. eee ney Ree 72 
PART IV. THE BILAMINAR BLASTOCYST 
General description............... SELLER EEE ese Soh ie seals onlee eee 73 
geekhemmatertali ss ac scene MEGAMI ined 5 ls 2 gee 73 
b. The development as seen in the living eggs.........................- 73 
he just completed bilamimar blastaeyste yess sere 6 85 &. os uns ee 76 
Tia Dhe embryonic ectoderm 72-5 Seen eh eee: ee 76 
ba Lhetrophoblasticiareay.c.04o eee COA Oe oe ee 78 
¢: Phe entoderm: ..52. 25 cate ets Oa 1d oe) oo ch ee 78 
‘The: t=1mm “blastocyst. o%, tistce ac. tee teres ee a eR ics, 3 tice 79 
as General desoription: 2.3 ations. ko ne mers ok Oar Es mee 79 
b. The bilaminar blastocyst according to Belen yi oy Sar a Ae eR 82 
c. /DheAl=mm. blastocyst according ton Miainotw-7,...21ae pe eee See ee 82 
‘Eheilatesbilaminar blastocyst parce 2044 eee eee Cer ee 84 
a. General-deseription(-s: -cc1oe een ae Bore ee ohare so. ee 84° 
by’ The: central light field in the embryonic area. .........2...-.62544.-) ee 85 
e. Modifiedentodermalicells;ee. 4. eee ee eee ark Cotiasateeecdc 86 
d. The ectoderm of late bilaminar blastocysts............ ae Buia EES 86 
é€. Yolk spherules in eetodermpandsentoderm:....-<. 5. 55..04:6.e ete eeee 87 
f,. Mesoderm tormationsiniguapedm. cen cesre ss a. eee ae ae ee 88 
UMN AT YF 3.8 Po ieee a ey As Se eins EE ea 2 Ck ee ee 89 
Tatera ture cited 2... on<4 oS Wee RTs OA One eon) oe She. ae een 97 
Explanation of fistires c's. (eee cs & ee ie pees ee eee ee 98 
Bates... Cecte btkcucae eee ee a Se ae Si Rae ee ne va See ee ee 99 
III. DESCRIPTION OF NEW MATERIAL ON MATURATION, CLEAVAGE, 
AND ENTODERM FORMATION 
INTRODUCTION 
a. Prefatory remarks 
The writer’s work on the development of the opossum began 
in 1912-13, when a preliminary study of the problem was made 
and the approximate breeding season determined for Austin, 
Texas. Active collecting was done in January and February, 
1914, and again in 1915, and the results of the study of the 415 
eggs secured from twenty females were published in March, 
1916. A considerable number of eggs, including several missing 
